Subject: Forensic Science

Topic: Bullet Comparison & Firearms Identification.

Chapter I

Introduction

This thesis deals with an important task within forensic science - the comparison of bullets for the purpose of firearm identification. Firearms identification involves the identification of a bullet, cartridge case, or other ammunition component as having been fired by a particular firearm. Such distinctive identification is made possible in this thesis owing to machining processes used in the manufacture of firearms. These unique imperfections are transferred to the ammunition in the course of its firing, making it possible to determine which firearm fired a specific bullet. Bullets bear groove-shaped marks that can be thought of as a kind of fingerprint of the firearm on their circumferential surface. In order to match an evidence bullet to the firearm that discharged it, the submitted firearm must be test-fired into a water tank to produce fired specimen bullet and cartridge cases for comparison with the evidence ammunition components. To enable a reliable feature extraction, various microscopic features of the bullets are compared. The performance of the methodology presented in this thesis is demonstrated and quantitatively assessed with an image database of real bullets. It is shown that with our methods, the efficiency of an automatic identification of firearms can be dramatically increased.

When identifying a fired bullet from a specific firearm both class and individual characteristics are used. Class characteristics are considered any intentional or design characteristics that would be common to a particular group or family of items. The class characteristics of firearms that relate to the bullets fired from them includes the caliber of the firearm and the rifling pattern contained in the barrel of the firearm. Rifling patterns consist of grooves cut or formed in a spiral nature, lengthwise down the barrel of a firearm. If these class characteristics agree the next step is to attempt to make a positive match between the individual characteristics that transferred to the bullet from the barrel. Individual characteristics rise from imperfections on the surface of the interior of the barrel that leaves microscopic striations (scratches) on the fired bullets. Striations have the potential to be consistently reproduced in a unique pattern on every bullet that passes down the barrel of a firearm. When these markings on a bullet are poor or disfigured a match between a fired bullet and a weapon can be extremely hard to make, if not impossible. . This research focuses on the microscopic comparison and potential identification of fired bullets as having been fired from the barrel of the same firearm.

Background of the Study

Development of Forensic Science through Ages [DFSTA] (2008) says “Crime in some form or the other has existed since the beginning of human race”. Fisher et al (2006) expands by saying, crime involving the use of firearms is more prevalent in today’s society. Firearms are regularly used in homicides, suicides, robberies, assaults and other violent crimes. Oftentimes the perpetrator leaves bullets, cartridge casings, or even the gun itself at the crime scene. These items of evidence, when examined properly, can yield a great deal of information about what took place at the scene of the crime.
With the advancement in science and technology, the concepts of crime as well as the methods adopted by criminals in its commission have undergone a phenomenal change. On one hand the intelligent criminal has been quick to exploit science for his criminal acts; on the other hand the investigator is no longer able to rely on age old art of interrogation and methods to detect crime. In this context FORENSIC SCIENCE has found its existence.
The term forensic is derived from the Latin word “forensis” which means belonging to courts of justice or to public discussion and debate. It therefore means the science which is used in courts for justice (Cork et al., 2008).
In the Handbook of Forensic Science, the Federal Bureau of Investigation (1981) defines firearms identification as “the study by which a bullet, cartridge case, or shot-shell casing may be identified as having been fired by a particular weapon to the exclusion of all other weapons” (Cork et al., 2008).
In the early part 1900—1930, according to Juarez & Pepin (2007), the science of firearm and toolmark identification was recognized by numerous judicial (law) systems in several countries around the world (cited in Cork et al., 2008). Legal recognition was due, in part, to the efforts of several individuals from various countries around the world that had conducted research and experiments into the identification of fired projectiles and cartridges cases to the specific firearms.
That beginning of nineteenth century, physicians were allowed to state their medical opinions in court. Since then, the courts have been slow to accept the opinions of other experts, and to recognize other sciences. The courts will not allow the testimony of polygraph examiners, hypnotists, and graphologists.
During this crucial period, the future of Criminalistics was threatened by phony experts. Perhaps the most serious threat involved one of the era's most famous personalities, the star witness in the Sacco-Vanzetti defense, and a key player in dozens of celebrated cases. This man was Albert Hamilton, and, as it turned out, was America's greatest courtroom charlatan.
For decades, direct comparison of bullet and cartridge case evidence has been used to link crime incidents to other crime investigations to link specific pieces of evidence to each other and to particular weapons (Cork et al., 2008).
Firearms themselves have had a long, illustrious, and documented history, while the first written reference to the subject of firearms identification has been recorded as occurring in 1900 with Hall’s “The Missile and the Weapon” in the Buffalo Medical Journal (Becker, 2004). It was not until the 1920’s, however, that the topic gained attention. Calvin Goddard, often credited as the “father” of firearms identification, was responsible for much of the early work on the subject during his examination of the various kinds of firearms and bullets at his Scientific Crime Detection Laboratory in Chicago (Becker, 2004).
In 1835, a homeowner was shot and killed, and the servant was suspected of the crime. Henry Goddard, one of the Bow Street Runners (an early London police force), investigated the case. By thorough examination of the physical evidence, Goddard was able to identify a visible flaw in the fired projectile and trace the mark to the manufacturer’s mold. He also identified the paper patch used to provide a seal between the projectile and the gunpowder as having been torn from a newspaper that was found in the servant’s quarters.
In 1900, a significant article titled “The Missile and the Weapon” was published in the June issue of The Buffalo Medical Journal (Becker, 2004). The article, Becker (2004) continued, written by Dr. Albert Llewellyn Hall, dealt with a variety of issues including the procedures for measuring land and groove markings on bullets. He also discussed the examination of gunpowder residues in barrels of firearms and the changes within the barrels that take place over time after firing.

Problem Statement

Heard (1997) in cases involving the use, or suspected use, of a firearm, every aspect of the firearm or ammunition has strong evidential value — but what to look for, what any evidence found actually means and how compelling that evidence is can involve very technical elements which are confusing. While startling advancements in modern scientific analysis have allowed forensic laboratories to produce sophisticated results, these advancements have also increased the gulf between the expert and the layperson who is required to interpret the findings.
The evidence at a crime scene can vary greatly in size, type, and physical structure. No matter what you’re presented with, though, it’s up to you to capture all of the evidence you find and maintain its integrity; if you fail to do so, you may jeopardize the entire case when it goes to court.
Maintaining the integrity of the evidence requires that you preserve it in the same condition in which you found it. To accomplish this task, you first have to choose packaging that is the proper size and material to fit the evidence. The bottom line is that the evidence collected at the scene, at the hospital, from the suspect, or from the victim could make or break the case. Certain pieces of evidence will degrade or otherwise change if not handle them properly.

Purpose and Objectives of the Study

This study focuses on how investigators used their knowledge of firearms identification to find the perpetrator of the crime. Its primary objective is to investigate the variables such as the origin of the fired bullet, impact damage, the environment, and the collection and preservation of bullets. And secondary objectives are to describe the procedures employed at a crime scene in packaging firearm or tool mark-related evidence; to define chain-of-custody procedures for evidence handling within laboratories; describe the differences between class and individual characteristics; to determine the length of time for which a bullet can be exposed to blood before any class and/or individual characteristics on it are degraded to the point where a positive identification can no longer be made; to describe the methods by which blood-exposed bullets are collected and packed at the crime scene were investigated – the aim was to determine the strengths and weaknesses of the methods so as ultimately to make recommendations for improvement; and to explore how investigators internationally collect bullets exposed to blood from crime scenes.

Rationale of the Study

Over the last decades, the importance of technical and scientific evidence for the criminal justice system has been steadily increasing. Unfortunately, the weight of forensic evidence is not always easy for the Trier of fact to assess, as appears from a brief discussion of some recent cases in which the weight of expert evidence was either grossly over- or understated. Also, in recent years, questions surrounding the value of forensic evidence have played a major role in the appeal and revision stages of a number of highly publicized criminal cases in several countries, including the UK and the Netherlands. Some of the present confusion is caused by the different ways in which conclusions are formulated by experts working within the traditional approach to forensic identification.

Definition of Terms

Ballistics – the study of projectiles in motion.
Caliber – is the diameter of the bore of a gun.
Cartridge cases – that part of the cartridge that contains the powder primer. It is the part of the cartridge that accommodates the projectile (bullet).
Circumstantial evidence – is all evidence other than eyewitness testimony.
Class Characteristics - characteristics used to identify the firearm, including caliber, direction of twist of the rifling, degree of twist, number of lands and grooves, and width of lands and grooves.
Comparison microscope – microscope that can be used to examine two or more fired bullets that exhibit the same class characteristics and determine whether the bullets were fired by the same weapon.
Degree of twist – the grooves inside the barrel of a gun spiral at a particular angle, with some steeper (greater) than others. It is an identifying characteristic of the firearm that includes the number of twists, the angle of the twist, and whether or not the twist is to the left or to the right.
Etching agent – agent used to visualize serial numbers on a firearm that is corroded.
Firing pin – the part of the firearm’s action that strikes the cartridge primer in order to fire it.
Grooves – the recessed areas between the lands of the rifling.
Identification - is the process of comparing two similar objects and concluding that both have a common origin.
Jacketed bullets –bullet that consists of a lead core surrounded by a jacket of harder material.
Lands – the raised ridges of the rifling that bite into the surface of the bullet and give it a rotational motion as it moves down the barrel.
Physical evidence – is evidence that can be handled, examined, tested, seen, felt or tasted.
Projectile – a solid projectile comprised normally from metal and often in the shape of a pointed cylinder or ball discharged from a firearm or air gun.
Rifled firearms – typed of firearm whose barrel has a set of spiraling lands and grooves.
Striation Marks – is a parallel surface contour variation on the surface of an object caused by the combination of force and motion, where the motion is approximately parallel to the place being marked?
Testimonial evidence – evidence that encompasses the testimony of witnesses and defendants.
Trace evidence – evidence such as blood, hairs, fibers, wood splinters, glass particles, paint, concrete, or soil particles that can be picked up by a bullet after it is shot and can indicate the items it traveled through.
Transfer - the process whereby two objects that meet leave evidence of their meeting, in particular, refers to the fact that a person entering and leaving a crime scene leaves something and takes something.

Research Hypotheses

Hypotheses specify exactly what is to be investigated. They are not the broad goal of the research, but specific things that must be observed, measured and interrogated to shed light on the broader topic. Hypotheses or research questions express the basis for the design (Denscombe, 2002).
Hypothesis 1: There will be differences in the amount and quality of striations and impressions made during the firing event.
Hypothesis 2: Improper packaging of bullets in various containers causes damage to the evidence.
Hypothesis 3: Damaged bullets or fragments of damaged bullets influence microscopic examination.

Summary of Remaining Chapters

The remaining Chapters of this thesis include a Literature Review (Chapter II) which records the firearm evidence; the chain of custody, collection, preservation and packaging of firearms evidence; firearms ammunition, and serial number restoration. Chapter III reviews the methodology of the research study and Chapter IV is a presentation of the results of the research related to the hypothesis given above. The final chapter, Chapter V, includes Discussions, Conclusions, and Recommendations based on the results.

Chapter II

The ability to compare bullets by examining microscopic striations on each bullet’s surface is at the heart of ballistics assessment. As stated before, microscopic striations are formed on a bullet’s surface during the firing sequence. Some causes of this include structural imperfections of the firearm or pressure created during the firing sequence. It has therefore been thought possible to “match” one bullet to another by firing both bullets from the same firearm.
Nichols (1997), in his exhaustive review of firearm and toolmark identification literature, examined thirty-four articles dating from 1949 to the present. Empirical studies conducted on bullets and casings fired through the same weapons have made up the majority of research. Nichols reports the earliest empirical study on firearm identification to have been conducted by Churchman in 1949.
Churchman (1949) analyzed characteristics typical of the Cooey .22 caliber rifle barrel. He emphasized the importance of knowing the origin of markings on bullets before one could utilize them for the purposes of unequivocal identification. The Cooey rifle was manufactured using the broaching technique, which Churchman believed was responsible for producing sub-class characteristics on the bullets (striations at the edges of the land impressions). He examined test-fired bullets from three consecutively broached rifle barrels. He found that the broach characteristics persisted from barrel to barrel. However, he also found individual characteristics of each rifle that did not carry over to the other two.
Skolrood (1975) conducted a study similar to that of Churchman. He performed a series of comparisons on bullets fired from three new, consecutively broached, .22 caliber Winchester rifle barrels. He found that comparisons of bullets fired from the same rifle yielded more persistent characteristics than comparisons on bullets fired from different rifles. Thus, bullets fired from a specific gun had a higher matching rate than bullets fired from other guns of the same type and manufacturer.
Freeman (1978) conducted a study on three consecutively rifled, Heckler & Koch, 9-mm Luger caliber, polygonally rifled barrels. He found that each barrel was distinctly individual, and that, although the first two barrels could be easily inter-compared, the third barrel yielded poorly marked test bullets. Thus, even consecutively rifled barrels contained individual characteristics, even though they were manufactured one after the other.
In contrast to the previous studies, Matty (1985) conducted comparisons on three revolver barrels all cut from the same section of rifled tube. He had observed that the buttons used to rifle the barrels did acquire “some damage” and wanted to see if the damage was transferred to the bore surface. Matty did observe longitudinal striations on the groove impressions caused by button imperfections, of which a few persisted along the length of all three barrels. He found that there was a settling-in period during which test fired bullets from the same barrel could not be identified to each other. This was important because of the question of how similarity between bullets could be proven with newly manufactured guns. Matty also found that, after the settling-in period, comparisons of bullets fired from different barrels proved inconclusive for groove impressions and showed no consistency for land impressions.
Brown and Bryant (1995) compared barrels from multi-barreled derringers in an attempt to determine whether the barrels in these weapons may have been consecutively manufactured. Brown and Bryant (1995) indicated that, “a major contributor to the individual bullet striation from the button rifled barrels is certainly the compressed reamer marks that appear very prominently in the casts of the lands and grooves” (p. 256). This meant that the marks transferred as individual markings to the surface of the bullets and would not be considered class characteristics but individual characteristics that could show consecutiveness among bullets fired from a single gun.
The need for a standardized, highly accurate firearms identification system has been shown throughout the history of firearms identification. During the early parts of the twentieth century, a magnifying glass was the tool most often used in the examination of firearms and bullets. This method did not last long, for the advent of the comparison microscope made possible photographs of two bullets showing similarities and differences.
Miller (2001) used a comparison microscope in his experimental research study and explains the process by which striation marks are produced in the firing process. He demonstrated in his studies that striation marks are of the utmost important to firearm examiners when they conduct microscopic comparisons on bullets.
In like manner, Inbau (1999) detailed the workings of this system of identification. The comparison microscope consists of two ordinary microscopes arranged in a way that images passing through both are brought together in one eye-piece midway between them. Each bullet (a test bullet and the suspect bullet) is placed under each lens and, by properly focusing the instrument and placing the bullets in the same orientation, the microscope transmits the fused picture of the two bullets. The two pictures were merged together as one. If the two bullets were fired from the same weapon, there would be very little difference between them in the way of markings and striations. This was an innovative technique in its day, as it was possible to make a visual inspection of two bullets at the same time.
Unfortunately, this system contained flaws in the accuracy of the picture projected and the ability of an “expert” to make a decision concerning the matching of a gun and bullet. More sophisticated and faster paced techniques were needed to accumulate the ever-growing number of comparisons to make. The laser topography system is one such innovative technique.
A study published by De Kinder et al. (1998) introduced a new technology for firearms identification – laser topography. The problems with the previous system of comparison microscopy are differences in light intensity (global or for different regions of the object under study), the surface material (nickel or copper), type of light used (temperature of the light source), and angle of incidence of the light (how light hit an object in order to be reflected back).
As a bullet is propelled from a rifled barrel – that is, one having the spiralling set of grooves and lands that constitute rifling – a series of parallel markings are imparted to it. These are the land impressions and groove impressions that constitute class characteristics (which will be similar to those on other firearms of the same make and model) and also certain parallel scratches. These striations, as they are known, are the result of imperfections in the bore – tiny flaws in the lands and subsequent use. These striations represent individual characteristics that make it possible to match a test bullet with a questioned one and demonstrate that they came from the same firearm to the exclusion of all others (Swanson et al., 1988). The class characteristics are useful in identifying the type of bullet. The number of lands and grooves in firearms, which vary from two to twenty-two, can help in identifying the make and model, as can the widths of these impressions. The direction of spiral of the rifling is also important. Rifling with a left-hand twist is often called “Colt-type rifling”, while that with a right-hand spiral is often referred to as “Smith and Wesson-type rifling”. The degree of twist occasionally may be useful as well (Rowe, 1982).
After the firearms expert has determined that the class characteristics of the markings on the questioned bullet are comparable with those of the test bullet (which is fired into boxes of cotton waste or a special “recovery tank” filled with water), a comparison of the two is made with the comparison microscope. This is basically two compound microscopes joined by means of a comparison bridge, which consists of a system of lenses and mirrors that permits the two images to be viewed side by side, with the field of view divided equally (Rowe, 1982).
The comparison may be recorded on one of more photomicrographs. It should be borne in mind, however, that even if bullets were fired in succession from the same weapon, not all individual characteristics would be identical. There would be some striations caused by powder residues, rust, corrosion and pitting, sand or dirt, and other surface factors or “fugitive” materials which of course are not likely to be duplicated on all bullets fired through that particular barrel. Moreover, there might be other striations on the bullets which would have no relationship to the interior of the barrel through which they were fired. For instance, there might be marks on metal-cased bullets due to imperfections on the interior of the sizing die used in the fabrication of the bullet. Likewise, fired bullets might contain crimp or burr impressions left there by the mouth of the cartridge case or shell. Obviously, the presence or absence of such marks, whether duplicated or not, must be discounted by the firearms identification technician (Inbau et al., 1972).
If the entire process seems somewhat laborious and painstaking, it is. Therefore, firearms technicians welcome a new development in the field: state-of-the-art computer systems and software that enable crime laboratories to trace bullets and shell casings through the identification of their identifying characteristics. Programs with names such as “Bullet-proof” and “Drugfire” do not actually take over the work of the firearms expert; rather, they function as sorting aids. Markings on the bullet or shell are typically recorded by microscope and video camera and then are digitally translated for computer storage. The computer can subsequently report those files with similar characteristics so that we can then manually compare them and perhaps make a match. The bullet-proof database can store and retrieve data on three hundred thousand bullets and compare fifty-five thousands in less than two hours – work that would require years by the manual methodology. It has been estimated that the program can perform the work of fifty firearms technicians (Benson, 1995).
The individual characteristics that a barrel imparts to a bullet may be destroyed by rust, corrosion, or the firing of thousands of rounds of jacketed ammunition down a barrel. Accumulation of large quantities of dirt and grease from multiple firings may also alter to some degree markings imparted to a bullet.
If a bullet with a diameter smaller than that intended for the specific weapon is fired down by a barrel, the bullet will be unable to follow the rifling sufficiently to produce repetitive markings. Comparisons cannot be made, as it is highly unlikely that two bullets will “slip” down the barrel in the same identical manner.
If the bore of the weapon is severely rusted, it is possible for serially fired bullets to have different markings. These occur because each bullet strips off rust and changes the surfaces of the grooves and lands.
The material, from which the bullet is constructed, and the velocity and pressures of which the cartridge is loaded have an effect on bullet markings. Therefore, it is good practice to use the same brand of ammunition as that fired from the suspect’s gun when trying to make a comparison. In fact, it is best to use other cartridges taken from the gun or from the same box of ammunition for comparison testing. The reason for these suggestions is that ammunition may vary greatly from one lot to another. The bullets used in one lot may be slightly different in composition from those of another lot. The powder used may be completely different, and the cartridges may be loaded to a higher or lower pressure. Because of all these variables, it is always best to try to obtain ammunition from the same lot as that from which it is to be compared.
Bullets that have been fired from revolvers may show skid marks when examined under the comparison microscope; that is, the grooves on the bullet are wider at the nose that at the base. Skidding occurs when the bullet jumps the gap between the cylinder and the barrel and stroked the lands. The bullet resists the attempt of the lands to impart a spin and “skids”. Bullets fired from an automatic may show skid marks when the bullet is slightly smaller than the desired diameter for a particular bore. This discrepancy causes the bullet to skid as it enters the rifling before settling down. As a general rule, however, skidding rarely occurs in automatic pistols, as the bullet is in contact with the lands before firing and follows them from the start. The presence of skid marks on an automatic pistol bullet may be of significance, as it may indicate that the bullet was fired in a revolver rather than in an automatic. Revolvers have been designed and manufactured to fire the .25 ACP and the .32 ACP pistol cartridges and have been and are still being manufactured for the 9-mm Parabellum and .45 ACP cartridges. Some revolvers have been chambered for rifle cartridges, e.g., .30 Carbines, etc. Revolvers have been and still are manufactured with the capability of firing different calibres by changing the cylinder, e.g., 38/9-mm, .22LR/.22Magnum.
Semi-automatic pistols have been chambered for revolver cartridges, e.g., .32 Smith & Wesson Long, .38 Special, .357 Magnum, .44 Magnum etc.; derringers for semi-automatic pistol cartridges (.25 ACP, .32 ACP, 9-mm Parabellum, .45 ACP, etc.), rifle cartridges (.22 Hornet, .223, .30-30, etc) and shotgun cartridges. Single-shot weapons have been chambered for a host of revolver, pistol, and rifle cartridges. Rifles have been chambered for hand-gun cartridges. Double-barrelled rifles have been produced with one barrel chambered for a rifle cartridge and the other for a shotgun shell.
In some instances, bullets may appear distorted when recovered from a body due to the fact that they were fired in weapons not chambered for them. Ward et al reported two cases, a homicide and a suicide, in which .38 special wad-cutter cartridges were fired in .30-30 rifles. The .38 Special cartridge will chamber and fire in this rifle, though the case expand and usually burst. The diameter of a .38 Special wad-cutter bullet (.358 inches) is significantly greater than that of a .30-30 bullet (.308 inches). On firing, the .38 Special is swaged down to the bore diameter of the rifle resulting in an elongated bullet, the diameter of the bore, with prominent lands and grooves.
Bullets recovered from decomposed bodies may show partial or complete loss of individual rifling striations depending upon the tissue from which the bullet was recovered and the constructive methodology of the bullet. In an experiment to determine the effect of decomposition on bullet striations, Smith et al. inserted bullets, of various constructs, into different areas of a human body and let it decompose for 66 days (Fackler, 1988). They found:

1. Nylon-clad bullets were uniformly unaffected by decomposition.
2. Aluminium jacketed bullets were mildly affected but there was no loss of striations
3. Lead bullets from the brain, chest cavity, and abdominal cavity showed mild tarnishing but were match-able while those from fat and muscle showed dissolution and oxidation to the point of impairing a match.
4. Bullets with copper alloy jacketing, including those with nickel-wash, were not match-able except for cooper alloy bullets recovered from chest cavity which were borderline.

In collecting firearm evidence, care should be taken not to place accidental markings on soft lead bullets (as from the unwise use of forceps). Also, no foreign object should ever be inserted in the barrel of a gun (despite the practice of TV detectives who use a pencil to pick up a pistol so as not to leave fingerprints). Such a practice may scratch the inside of the barrel, thus altering the striation evidence that will subsequently be produced on test bullets. Tissue, blood, fibres, or other trace evidence may be dislodged as well (Swanson et al., 1988).

Preservation and Collection of Firearms

The officer recovering the gun should carefully examine the weapon and prepare an evidence tag at the crime scene that contains a full description of the firearm, including the manufacturer’s name, the caliber, the model and the serial number. The tag is usually attached to the trigger guard. If the firearm is not a common model, additional marking may be found on the frame or barrel of the weapon. These letters and symbols may prove helpful in identifying the weapon’s origin (Girard, 2007).
If the firearm was loaded, the number and type of rounds should be noted. When a revolver is recovered, each chamber should be examined and marked. A diagram should be made with each chamber being designated with a number; as each cartridge or spent casing is removed, its position should be marked on the diagram. Information about the positions of spent cartridge cases in the chamber may be useful in establishing of sequence of events later. Each round removed should be placed in a separate labeled paper envelope with a label that references the chamber number from which it came.
Once the firearm has been safely unloaded and properly tagged, it is ready for transport to the crime laboratory, or to the evidence custody location. When custody is passed to another location, the seizing officer should get a receipt for the evidence to establish a proper chain of custody.
Weapons that are found underwater should be transported to the lab in a container filled with water from the same source to minimize rusting of the gun.

Chain of Evidence

Chain of custody ultimately applies to the handling and documentation of evidence. According to Siegel et al. (2000), the chain of custody process is used to authenticate evidence. Continuity accounts for the custody of evidence from the time it is identified at the scene until its presentation as evidence in court. In the case of a firearm seized at the scene, the make, model, serial number, and condition must be documented together with the name of the person who collected and packed the firearm. It is noted that all transfers, such as the date, time and person who handles the firearm must be documented throughout the investigation.
“If continuity is broken, if one link of the custody is lost or weak, the value of the evidence is called into question and may be pronounced inadmissible” (Greenshield & Scheurman, 2001). In the case of firearm seized, if the condition of the firearm change (e.g. if the firing pin breaks) and the change cannot be explained, one may conclude that the evidence was tampered with and may result in the evidence not being admitted in court. Any weak link in the chain admits the possibility of accidental contamination or deliberate tampering, and provides the basis for the defenses of reasonable doubt (Axelrod & Antinozzi, 2003).

Ballistics

Ballistics is the study and science of the passage of projectiles in motion and may be divided into three subsets. According to Dodd (2006) the three subsets of ballistics are internal, external and terminal.
Internal Ballistics: Heard (1997) defines internal ballistics as a study of what happens inside the weapon from the moment the firing pin strikes the primer to the time the bullet exits the barrel. It is mainly concerned with the propellant pressure that forces the bullet from the cartridge case down the barrel of the weapon.

Fig 1 Rifling inside a barrel (Wilson, 2006)
The motion of the bullet down the barrel of a weapon is a process in which rifling marks, illustrated in Fig. 1, which “consist of lands and grooves” (Wilson, 2006), are impressed on the bullet. Fig. 2 shows a pristine bullet containing rifling impressions (Thomas, 2005). According to Warlow (2005), finding these markings with other features will allow the firearms examiner to provide police agencies with valuable information.
Rifling is the term given to the spiral grooves cut into the bore of a barrel which imparts a stabilizing spin to the bullet. This spin keeps the bullet traveling in a point - first direction and lessens any tendency for it to depart from its straight line of flight. As such, this was a very significant event in the evolution of firearms.
Girard (2007) stated that rifling is slightly different for each weapon, because different gun manufacturer use different spiral grooves. Given that the rifling imprints different scratch patterns on the bullet, the forensic scientist can use these patterns to identify the manufacturer of the weapon used in a crime. As the broach cutter is pushed through the barrel to cut the grooves, it liberates shards of steel from the side of the barrel. Depending on how worn it is, the broach cutter may also produce fine lines called striations, that run the length of the barrel in the lands of the barrel.
Striations are formed either from imperfections on the surface of the broach cutter or from steel shards that become wedged between the broach cutter and the barrel wall. The random distribution of striations in the wall of the barrel is impossible to duplicate in any two barrels. Even barrels made in succession will not have identical striations. Because no two gun barrels will have identical striations, the individual marks left on bullets by the gun’s striations will produce individual characteristics that will allow a forensic examiner to match a bullet to the gun barrel it was fired from.

Fig. 2: Rifling impressions on a fired bullet (online)
External Ballistics: Wilber (1977) describes external ballistics as the actual path that the bullet follows before it impacts the target, also known as trajectory. This phase of firearms examination starts when the bullet exits the muzzle of the firearm. Most people believe that the path of a bullet is straight, which is in fact untrue. Fig. 2.3 shows the principal aspects of the trajectory of the flight of a bullet, which is parabolic in shape.
Girard (2007) describes it as the events that occur after the bullet leaves the barrel of the gun but before it strikes its target. An understanding of external ballistics will help investigators understand where the perpetrator when he or she fired the gun. The bullet may have traveled in a straight path to its target or it may have ricocheted as a result of striking another object. If it can be proved that the victim died from a ricochet rather than a shot aimed at the victim, this evidence may convince a jury that the perpetrator is not a murderer.

Fig. 3: The barrel’s angle in relation to the flight path of a bullet (online)
     Terminal Ballistics: Warlow (2005) states that the damage to a bullet depends on the bullet mass, velocity, design, construction and the nature of the target.
Girard (2007) says terminal ballistics ultimately deals with what takes place when the bullet strikes the target and the effects this has on one another. It refers to what happens when the bullet strikes its target. When a bullet hits a person, it enters the body, forming an entry wound. If the caliber of the weapon used is large enough, the bullet will have so much kinetic energy that it will not be totally stopped upon entry. Instead, it will pass through the victim, leaving an exit wound. A careful examination of the wounds will establish whether the bullet pass through the victim from front to back, or vice versa.
     During this brief period of interaction with the target, the bullet suffers some degree of deformation and accumulates deposits. Fig. 4 illustrates various degrees of damage to a fired bullet. Heard (1997) agrees that damaged bullets or fragments of damaged bullets influence microscopic examinations.

Fig. 4 Multiple fragments of fired bullets
(online)

The condition of the bullet and the amount of rifling left on the bullet is a major factor in microscopic comparisons” The worksheets reviewed from case files revealed that bullets are received in various shapes, forms, and sizes. The complete circumference of the bullet comes into contact with the barrel, and it is the complete circumference of the bullet that is the true representative of the barrel. If all the lands and grooves are not present, the examiner may find it difficult to make microscopic findings.
When bullets are fired through bodies, one possibility is deformation as bullet travels through tissue, which relate to terminal ballistics (Thomas, 2005). The other damage is from chemical reaction causing corrosion. This means that degradation could change, alter and/or distort striation marks on fired bullets required for bullet comparison.

Automated Systems

The continuous evolution of smaller, more powerful computers since the 1990s has heralded the arrival of a powerful screening tool for firearm identification experts. Automated “search and retrieval” systems have the objective of enabling the comparison of evidence and control bullets, therefore “transforming forensic ballistic analysis from an evidence verification tool into a crime-fighting tool” (Bachrach, 2002).
The history of forensics and firearm identification in particular has shown the need for a comprehensive system of comparing evidence bullets with test bullets in order to match a suspect gun to a shooting. The creation of striations and impressions on a bullet’s surface during the firing process allows for an examination of whether the striations and impressions are consistent among bullets fired by the same gun. The advent of computers has allowed for faster, more comprehensive processing of striations and impressions on a bullet’s surface than the original comparison microscope did. The large quantity of guns being used in shootings shows the need for a database of firearm and bullet characteristics.
When bullets are matched perfectly, there will be a correlation of 1.0. As there are many types and manufacturers of bullets, it remains to be seen whether bullet types are affected by or themselves affect the striations and impressions created on bullets during the firing process.

Chapter III

Firearms Identification is a comparative examination, where the ammunition components of unknown origin (from the scene of the shooting or body) are compared with bullets, cartridge cases, and shotshell of known origin that have been produced in the laboratory by test firing the suspect firearms.
A submitted firearm is fired several times using a water tank approximately 3 feet wide, 10 feet long and 3 feet high (Doyle, 2007), to obtain standards from the firearm. Lids on the tank are closed and locked and the muzzle of the firearm is placed in the open tube at the end of the tank and fired. Friction from passing through the water slows the bullets down and they end up on the bottom of the tank about halfway down its length (Doyle, 2007).

Research Hypotheses

Hypothesis 1: There will be differences in the amount and quality of
striations and impressions made during the firing event.
This discipline in the laboratory is largely responsible for examining fired bullets, cartridge casings, and shot-shells. A variety of markings or impressions are left on the items when a firearm is discharged and the markings provide points of comparison to other ammunition fired from the same weapon.
Firearms identification involves the identification of a bullet, cartridge case, or other ammunition component as having been fired by a particular firearm (Schehl, 2000). The methodology normally used in bullet identification involves a comparison of the evidence bullet and a test bullet fired from the weapon. The two bullets are then compared by means of a comparison microscope, which permits a split-screen view. This allows for visual identification of striations and other marks.
The bullets to be compared are attached to short cylindrical bullet holders. The bullet holders slip onto the shafts of the bullet-manipulating mechanisms, which in turn are attached to the microscope stages. The bullet-manipulating mechanisms are provided with universal joints so that the bullets may be oriented at any desired angle (Becker, 2004).
     Becker (2004) continued, once the bullets are mounted, the examiner begins the search for matching patterns of striations. A land impression on a test-fired bullet is compared successively to each land impression on the suspect bullet until a match is obtained or it is determined that no match is possible. Once a match has been made with a pair of land impressions, the bullets are rotated synchronously to see if other matching striation patterns may be observed.
Given that there are many manufacturers of bullets, it seems appropriate to hypothesize that there will be differences in the amount and quality of striations and impressions made during the firing event. The reason behind this concerns the differences in the quality and manufacturing processes of ammunition today. Some manufacturers have sophisticated high-tech processes that create identical bullets, while other manufacturers may not have such high standards. In other words, bullet manufacturers will make a difference in the ability of a bullet to acquire striations during the firing event.
I shall utilize a stereo-microscope and comparison microscope during examination. When examining a bullet or casing from a crime scene, the stereo-microscope has provided general information about the item and revealed trace evidence such as blood or fibers. The comparison microscope designed for firearms examinations is equipped with special stages to hold the evidence in a variety of positions and also provides an external light source.
The firing tank, which is a large, study container of water, is used to collect fired bullets. The fired bullets can then be compared to bullets from the crime scene. Conclusions regarding this comparison depend on the amount and quality of the striations and other markings present on the unknown bullet. Another device used is a firing target that allows study of gunpowder or pellet patterns produced at various distances from the target. Distance determinations are commonly necessary when a person claims that a shooting was in self-defense and occurred during a struggle. Identification of residue pattern and gunshot residue by chemical tests can also assist in a shooting reconstruction.
Impressions made by tools can also include striation-type-markings. Tool marks are commonly examined in the firearms sections. A burglar who uses a screwdriver, bolt cutter, or other common tool may leave impressions of the tool at the crime scene. The item containing an impression or a cast of the impression is collected and when the suspected tool is located, an impression is made by the tool and compared to the crime scene evidence. The burglar’s screwdriver is a common example of tool mark evidence, but any tool can create impressions on a variety of surfaces. Knife wounds to flesh and bones and cutting tools used to cut wires, cords, and bindings are only a few examples of other tool marks that may be of value.

Hypothesis 2: Improper packaging of bullets in various containers causes
damage to the evidence.
Two spent bullets were packed in two separate containers. One in ordinary plastic containers and plastic zip-lock bags and the other one in a specimen cup wrapped in towel or in envelopes. Wilson (2006) found that fired bullets packed in ordinary plastic containers and plastic zip-lock bags had the most damage, while very little change was observed in bullets that were in a specimen cup wrapped in a paper towel or in envelopes.
“Spent bullets should be packaged in such a way as to reduce any jostling or friction that might damage the striations” (Flynn, 2005). They should be packed in soft cotton or tissue, sealed in a paper envelope or pillbox and the container should be marked for identification. The bullet should not be marked due to the possibility of destroying valuable evidence. Plastic or glass airtight containers should never be used for bullets as they could allow moisture to cause corrosion of identifiable detail on the bullet. Paper or cardboard should be used. Bullets should always be handled with the utmost care to avoid destroying the microscopic striations on the bullet.
Like with all crime scenes, those that contain firearm evidence must be clearly documented and photographed. Because bullets and cartridges are small, they must be identified in photographs with a label or market of some type.
Bullets should always be handled with the utmost care to avoid destroying the microscopic striations on the bullet. They should be packed in soft cotton or tissue, sealed in a paper envelope or pillbox and the container marked for identification. The bullet should not be marked due to the possibility of destroying valuable evidence.
Plastic or glass airtight containers should never be used for bullets. They could allow moisture to cause corrosion of identifiable detail on the bullet. Paper or cardboard should be used.
Often, bullets or shotgun pellets may be found in walls or ceilings. The preferred collection method in such cases is to remove the section of the wall or ceiling and send it to the lab, where the bullet or pellets can be safely removed. If this is not possible, then rubberized tools must be used to remove the bullets.
Bullets and cartridges must never be marked for identification anywhere on their surfaces where there might be forensically significant markings. Many crime scene investigators do not mark them at all but put them in small vials or boxes, and then mark the containers. Likewise, weapons should never be marked in places where there might be evidence. Sometimes, tags can be used. Guns should never be handled by putting a pencil or anything else in the barrel. This could change the markings in the barrel and render test firings useless.
In many cases, firearms are coated with a thin layer of lubricating oil. This makes them unsuitable surfaces for retaining fingerprint images. Nonetheless, weapons should always be packaged in such a way that fingerprints could be collected if present.
I kept in mind that weapons may be a source of significant trace evidence and that the examination of the weapon may have to be put off until the trace evidence is processed. As previously mentioned, fingerprints are unlikely but not impossible to recover. Blood, fibers, or paint flecks may be on the weapon. Bits of tissue from a close-in or contact shot may be present on the weapon or inside the barrel. If the weapon were in the owner’s pocket, it may have picked up trace evidence such as lint, fibers, and dirt.
Once recovered, the weapon should be identified and as much information as possible should be gathered from the weapon. The serial number is especially important. Criminals also know this, and in many cases they will grind down or file off the serial number.

Hypothesis 3: Damaged bullets or fragments of damaged bullets influence
microscopic examination.
The barrels of modern firearms are "rifled," that is, several spiral grooves are cut into the barrel from end to end. As the bullet is propelled through the barrel, these spiral grooves and lands (the raised portions of the barrel between the grooves) set the bullet spinning around its axis, giving it rotational as well as forward movement, thus increasing its stability in flight. The lands and grooves consequently etch a pattern of very fine striated lines along the sides of the bullet, which will vary from one weapon to another just as fingerprints vary from one person to another. Like fingerprints, the lands and grooves scratched onto the surface of the bullet can be microscopically identified with a particular weapon to the exclusion of all others, provided that they remain sufficiently intact subsequent to impact (R547-48).
Shaving one surface of a bullet fired from a revolver is sometimes encountered. This happens because the cylinder of the revolver is improperly aligned with the bore of the barrel (so-called poor indexing) and thus lead is shaved from the bullet as it jumps the gap from the cylinder to barrel. Both cheaply made revolvers and revolvers of quality that are badly worn may cause shaving.
In some case, a bullet is recovered from a body is too mutilated to make a bullet comparison possible. In other cases, the bulk of the bullet exited, and only bullet fragments are recovered. In such cases, it is possible by using semi-quantitative or quantitative compositional analysis to link the recovered bullet to bullets from or known to be fired by a specific gun. Recovered bullets fragments can be linked to a bullet recovered from the scene or to bullets from or known to be fired by a specific gun. Scanning electron microscopy with energy dispersive x-ray (SEM-EDX) is suitable for semi-quantitative and inductively coupled plasma atomic emission spectroscopy for quantitative compositional analysis.

Chapter IV

The descriptive statistics for average weighted striations by bullet type are presented in Table 1. Minimum and maximum values, as well as the mean and standard deviation, are included. It should be noted that the number of test firings are not equal. The number of striations measured is larger than the number of bullet firings.
Table 1: Descriptive Statistics for Average Weighted Striations

 

Bullet Type	Number of bullets test fired	Number of striations measured (N)	Minimum	Maximum	Mean	Standard Deviation
MAG	730	4470	.240815	.925541	.45244033	.159813317
PMC	629	3870	.246069	.923105	.4469079	.155101147
RUMC	558	3270	.212806	.904019	.43961947	.130965913
WIN	424	2670	.241248	.937230	.45247036	.149304754
CCI	288	2070	.221234	.877064	.41355351	.115519092
NOR	225	1470	.237090	.865893	.44045690	.124976656
FAE	148	870	.288131	.954995	.46271168	.148342660
LB	47	270	.286378	.966533	.44350814	.127912088
All types	3049	18960	.212806	.966533	.44418845	.14519715

Normality Tests

The distributions of average weighted striations showed bi-modality, with a mean of around 0.4 to 0.5, and large amounts of numbers on either side. Therefore, the distribution was split in half to show the distribution of incorrect orientations, which were proposed to have significantly lower numbers than the correct orientation, which would approach 1. A cut-off point of 0.5 was used to separate the distributions into incorrect and correct orientations. As each bullet was measured using six orientations, only one orientation, with the highest number of striations, was deemed the correct orientation; the other five orientations were deemed incorrect and were therefore expected to have lower numbers than the correct orientation. Analyses therefore concentrated on examining differences among bullet types of the normality or non-normality of their distributions for both correct and incorrect orientations. The incorrect orientation distributions were proposed to approach a normal distribution for every bullet type, due to the low number of striations measured and the occurrence of measurement and random error. The correct orientations would be positively skewed, as they were expected to hover near 1. These show that all of the bullets by the same manufacturer were statistically similar in the number of average weighted striations.
The distribution on the left represents the incorrect orientations. A bell-shaped curve can be seen that almost straddles the middle of the graph. The histogram on the right, however, shows a correct orientation distribution. This histogram does not follow a curve, but does look leptokurtic. This is expected due to the higher numbers of striations for the orientation.
The analyses conducted were normality tests as well as histograms and Q-Q plots for each manufacturing type of bullet. The normality test conducted was the Kolmogorov- Smirnov test with a Lilliefors Significance Correlation. As predicted, most of the bullet type distributions, both correct and incorrect orientations, followed the hypothesized pathway. Tables 2 and 3 show the normality tests by bullet type and orientation.
Table 2: Normality Tests by Bullet Type for Incorrect Orientation

Bullet Type	Kolmogorov - Smirnov
	Statistic	Degrees of Freedom	Significance
MAG	.015	3740	.054
PMC	.026	3241	.000*
RUMC	.011	2712	.200
WIN	.024	2246	.003*
CCI	.020	1782	.096
NOR	.021	1245	.200
FAE	.025	722	.200
LB	.047	223	.200

* Significant at the .01 level
The Kolmogorov- Smirnov test shows that WIN bullets and PMC bullets measured in incorrect orientations do not follow a normal distribution, while the MAG, RUMC, CCI, NOR, FAE, and LB bullets do. Thus, six of the eight manufacturers follow the predicted pattern, while two do not.
Table 3: Normality Tests by Bullet Type for Correct Orientations

Bullet Type	Kormogorov - Smirnov
	Statistic	Degress of Freedom	Significance
MAG	.124	730	.000*
PMC	.090	629	.000*
RUMC	.054	558	.000*
WIN	.088	424	.000*
CCI	.049	288	.098
NOR	.107	225	.000*
FAE	.073	148	.050
LB	.150	47	.009*

* Significant at the .01 level

The results for the Kolmogorov-Smirnov test show all but CCI and FAE to be non-normal distributions, with LB near the cut-off point of .01. Thus, five of the manufacturing types followed the predicted pattern, while three did not. It is interesting to note that the manufacturing types with distributions differing from the expected path for correct orientations were not the same as those differing for the incorrect orientations. Implications of this will be discussed in the next chapter.

Analysis of Variance

Analysis of Variance tests were performed on each bullet manufacturer, testing across manufacturers as well as within all of the bullets of each manufacturer. The ANOVAs tested differences across the means of each manufacturer for the average weighted number of striations. Forty-five cases from each manufacturer were used, at forty-five was the lowest common amount (equal to the smallest group, LB). This was done to allow for further significance testing.
The ANOVAs for both the correct and incorrect orientations were significant at the .01 level, F (7, 352) = 50.798, p <.01 and F (7, 352) = 17.620, p<.01, respectively. These showed that significant differences existed across manufacturers for both orientations.

Post hoc Tukey and Homogenous Subsets Tests

Post hoc Tukey tests were then done to examine which manufacturers, if any, had significantly different means from the other manufacturers. Table 4 shows that there are significant differences across roughly all of the manufacturers at the .01 level.
Table 4: Tukey Test between Manufacturers for Correct Orientations

MAG	PMC	RUMC	WIN	CCI	NOR	FAE	LB
MAG	.682	.000*	.953	.000*	.000*	.702	.000*
PMC	.682	.000*	.999	.000*	.000*	1.000	.000*
RUMC	.000*	.000*	.000*	.000*	.362	.000*	.388
WIN	.953	.999	.000*	.000*	.000*	.999	.000*
CCI	.000*	.000*	.000*	.000*	.275	.000*	.254
NOR	.000*	.000*	.362	.000*	.275	.000*	1.000
FAE	.702	1.000	.000*	.999	.000*	.000*	.000*
LB	.000*	.000*	.388	.000*	.254	1.000	.000*

Table 5: Tukey Test between Manufacturers for Incorrect Orientation

MAG	PMC	RUMC	WIN	CCI	NOR	FAE	LB
MAG	1.000	.999	.000*	.120	.031	.000*	.003*
PMC	1.000	.999	.000*	.124	.030	.000*	.003*
RUMC	.999	.999	.000*	.409	.004*	.000*	.000*
WIN	.000*	.000*	.000*	.000*	.617	1.000	.951
CCI	.120	.124	.409	.000*	.000*	.000*	.000*
NOR	.031	.030	.004*	.617	.000*	.418	.998
FAE	.000*	.000*	.000*	1.000	.000*	.418	.848
LB	.003*	.003*	.000*	.951	.000*	.998	.848

As it can be seen by these results, significant differences exist across manufacturers. No two manufacturers were alike for both the correct and incorrect orientations. This was expected for correct orientations, as each manufacturer should have bullets that are not identifiable with other manufacturers. The weighted means should be significantly different; to show that the SCICLOPS system does not read every bullet as the same. For the incorrect orientations, however, there were some surprising findings. Although all of the manufacturers had one or more other manufacturers to whom they were similar, there were many more significant differences than expected. If the incorrect orientation numbers were due to chance, then there should not be significant differences across manufacturers.
The surprising lack of significant differences between manufacturers was also shown when conducting a Homogenous Subsets test, which examines the means for similarity and groups any similar bullet types together. These results are presented in Table 5 and 6.

Table 6: Homogenous Subsets for All Manufacturer Types in Incorrect Orientation

Tukey HSD

Bullet 1 Manufacturer	N	Subset for alpha = .05
		1	2
CCLid RUMCid PMCid MAGid NORid LBid WINid FAEid Sig	45 45 45 45 45 45 45	.30844982 .32125491 .32486969 .32494656         .120	.34438696 .34850109 .35533316 .35710813 .418

Means for groups in homogeneous subsets are displayed.
a.     Uses Harmonic Mean Sample Size = 45.000.

H5 - The first set of bullets was removed from their respective packages on June 14, after two weeks of being in blood. After thoroughly cleaning the bullets and examining them under the microscope, many intriguing results were found. The lead bullets had nearly no visible change, while the other three (copper, steel, brass) did. Another major observation was the difference in degradation amongst the different packaging. The samples of the regular specimen cup and the plastic zip lock bag saw the most change while the bullets remained in an aqueous environment. Those that were in the specimen cup wrapped in a paper towel and ones in the envelopes remained in dried blood and very little change was observed. Visible changes that occurred were pitting of the surface, formation of “craters and valleys”, and discoloration of the surface of the metal.

Chapter V

     Discussion
When a gun is discharged, two items are left behind that can identify that gun. One of them, as previously discussed, is the cartridge casing. The other is the bullet. To understand how a bullet can be used to identify a weapon, it’s really important to examine the barrel of a gun. All modern gun barrels, with the exception of shotguns, are rifled. Rifling is the process in which spiral grooves are cut into the length of the barrel. These grooves help the bullet spin so that it is stable in flight.
It has been observed that various degrees of damage were caused by various types of containers that range from pitting of the lands and grooves, discoloration of the metal and the deformation of striation marks. The following analyses were performed on the data set to address the previously stated hypotheses.
Hypothesis 1: There will be differences in the amount and quality of
striations and impressions made during the firing event.
Many published studies have demonstrated that no two firearms produce the same unique marks on fired bullets and cartridge cases – this is true even with firearms of the same make and model. The machining of the manufacturing process, combined with the use of the firearm, leaves surface marks on the metal parts of the firearm that are not reproducible in other firearms. These marks are transferred to the bullets and casings when discharged from the firearm.
Because there is no practical method of comparing the striations on the inner surface of a rifled weapon with the striations on a fired bullet, reference bullets of the same make, style, and caliber must be used, and striations created by firing them from the questioned firearm. Not only would cutting the barrel open be impractical, but also the comparison would then be between positive (the barrel) and negative (the questioned bullet) impressions.
The questioned and known bullets are first examined with the naked eye and slight magnification. The number of lands, grooves, their twist, and the bullets’ weight are recorded. Because these are higher order class characteristics, any deviations from the known bullet indicate that the two bullets were fired from different barrels. If the lands, grooves, and direction of twist all concur, then the next step is microscopical comparison of the striations on the bullets.
The comparison is performed on a comparison stereo-microscope with special stages that facilitate positioning the bullets in the focal plane and allow for rotation of the bullets on their long axis. The bullets are positioned on the stages, one on each, both pointing in the same direction, and held in position with clay and putty; this allows me easily reposition, and the soft material will not mark the bullets’ surfaces. The known bullet is then positioned to visualize a land or groove with distinctive striations. The questioned bullet is then rotated until a land or groove, respectively, comes into view with the same striation markings. The lands and grooves of the two bullets must have the same widths. More importantly, the two bullets must not be merely similar but must have the same striation patterns with no significant differences. This last point is critical: Not only must the forensic firearms scientist see the positive correlation between the significant information on the bullets’ surfaces; he or she must also not see any unexplained differences. I accept that each rifled barrel is unique: no two of them will have identical striation patterns. This is true even of barrels that have been rifled in succession, one after the other. It takes education, training, and mentoring to train a person’s eye and judgment on the subtleties of bullet striation patterns.
Hypothesis 2: Improper packaging of bullets in various containers causes
damage to the evidence. The groove depth of most rifling measures approximately 0.1mm (0.004 in). The high points in the bore are referred to as the lands. These effects are less easy to see or measure in polygonal pattern rifled barrels, but are evident as a regular pattern of distortion when the bullet is viewed at its base end. The barrels of the vast majority of commercially produced or modified firearms are ‘crowned’ at their muzzle ends, by means of rounding off both the inner and outer exposed edges. Revolver and pistol barrels are frequently cut to length from a longer piece of rifled tube or from a single forging intended to make up two pistol barrels. During all of these manufacturing processes small imperfections of a random nature are left behind in the bores due to tool wear and the nature of the cutting processes. These imperfections are added to by the effects of use and packaging, roughly handling or careless cleaning practices. It follows therefore, that a bullet passing through the barrel of a rifled arm will pick up both the normal intended pattern of rifling upon its exterior and also a pattern of fine lines or striations from the chance imperfections contained in the bore and at the muzzle. These imperfections and the normal pattern of rifling will of course be seen upon the bullet in a negative form. It has been established that such randomly produced imperfections, which can be seen under the microscope as a pattern of the fine parallel lines similar to supermarket packaging bar codes, are as unique as a person’s fingerprints. So I completely agreed that proper packaging of the bullet can protect the bullet or casings from scratching each other and thereby altering or obliterating the marks that I will examine for possible identification.
The known fired bullets must be captured, preserved, however, so that they are as “pristine” as possible and not deformed or damaged. Firearms are typically discharged into a water tank where the water slows and eventually stops the bullet without altering its striations; other bullet recovery systems are used from the simple (a bucket filled with rubber shavings) to the high tech (sandwiched layers of specialized materials). The known bullet is then recovered, labeled, and used as a reference in the comparison; multiple known bullets may be created, if necessary.
Hypothesis 3: Damaged bullets or fragments of damaged bullets influence
microscopic examination. The very fine lands and grooves along the copper sides of CE 399 allowed the conclusive determination that the bullet had been fired. After closely examining 399 at a magnification of five diameters, I was convinced of the veracity of the testimony. I followed each set of lands and grooves on the bullet and saw that all were continuous and without disruption, beginning just below the rounded nose and running smoothly down to the tail end (online).
No two weapons manufactured are the same. Every single one has marks in the barrel and breech and on the action that are unique. And because weapons are made from exceedingly hard "tool grade" steel, these unique marks leave unique impressions on every bullet and cartridge case cycled through them, all of which are made from softer metal than the weapon itself.
In the absence of the suspect firearm, the constructional makeup of the questioned bullet can play an important role in identifying its caliber, even if the bullet is damaged and class characteristic rifling marks are not identifiable.
A barrel will produce individual markings in addition to a bullet's land and groove impressions as the bullet passes through, and it is these unique markings that an examiner evaluates to determine whether a given bullet was fired from a particular firearm. Manufacturers use various cutting, swaging and electrolytic processes to introduce rifling into a barrel, and these processes, as well as others used in the finishing of a firearm, make each barrel unique (Wilson, 2006).

Conclusions
The following conclusions may be reached as a result of the comparison of bullets or cartridges:
The questioned bullet or cartridge was fired in the submitted weapon. This means that the class characteristics are consistent and that individual characteristics match. This is a positive identification.
The questioned bullet or cartridge was not fired in the submitted weapon. This implies that the class characteristics did not match. In this instance, no microscopical comparison will have been undertaken. This is a negative identification.
The results of the microscopical examination were inconclusive. This means that the class characteristics match, but that insufficient individual characteristics to declare a match could be found. The firearms examiner does not render a negative identification in this instance because the individual characteristics of the firearm may have been altered by further use. For example, there may have been bore erosion due to firing large number rounds through the weapon. The weapon may also have been deliberately altered by the shooter. Some weapons (e.g. Glock pistols) do not make usable striation patterns on fired bullets.
The microscopical comparison of bullets and cartridges is a very tedious and time-consuming process. Most law enforcement agencies keep open bullet and cartridge casing files where evidence from open criminal cases is kept for later comparison with test-fired bullets and cartridges from weapons recovered in subsequent investigations. However, the time required for such comparisons is such that they are attempted only when information developed by investigators provides a link between a new case and an open case.
Recommendations
Even if bullets were fired in succession from the same weapon, not all individual characteristics would be identical. There would be some striations caused by powder residues, rust, corrosion, and pitting, sand or dirt, and other surface factors or “fugitive” materials which of course are not likely to be duplicated on all bullets fired through that particular barrel. Moreover, there are marks on metal-cased bullet due to imperfections on the interior on the sizing die used in the fabrication of the bullet (cited in Tipton & Krause, 2007).
     Many of the same changes were also observed when the samples after four weeks in blood were examined. With these changes, none of them were serious enough to prevent the matching of a bullet to a standard in the two week samples but posed a serious problem in a few of the four week samples. Some of the individuals land impressions on the four week samples experienced enough degradation to where a match with the standard bullet could not be made.

However, identification between the sample bullet and the control bullet could still be made when based on other non-damaged land impressions. The entire surface of the bullets was not submerged in blood and some of the land impressions experienced little to no visible change.
     Samples from six weeks showed very minimal change and the bullets from eight weeks have not been fully processed due to my absence from the lab for military training. It should be noted that results from these bullets may be skewed due to the amount of blood they were stored in. When all of the samples were prepared the same amount of blood was not added to all of the samples. With this, some samples were almost completely submerged in blood and others (samples of weeks 6 and 8) only had a small portion of the bottom of the bullet being covered. The major problem in this is that most of the blood, even in the specimen cups and zip lock bags dried in the six and eight week samples. Dried blood does not degrade the surface of the bullets and these samples can not be used in determining the amount of degradation that occurs after resting in blood for six and eight weeks.
Rifling is produced in several different ways – it can be made by dragging a sharp hook through the barrel, or by pushing, hammering, or pulling a rotating cutting bit down the barrel. As the metal inside the barrel is scraped or cut away, grooves form. The original metal that is left untouched between each groove is known as a land. The lands and grooves therefore make up the rifling of the barrel. The diameter of the barrel, and therefore the caliber of the weapon, is the diameter between opposite lands.
Another important feature for discussion of rifling is the pitch. The pitch is the “steepness” or the angle of the grooves as they are cut into the barrel. The pitch of a barrel is usually expressed in what the length of the barrel would need to be the rifling to travel one inch around the circumference of the barrel. Rifling usually has a fairly shallow pitch so that the bullet can still quickly move down the barrel yet get enough spin from the lands and grooves for a straight flight.
Finally, the direction of the rifling is important as well, and this is known as the twist. The twist of a barrel can be one of only two possibilities: right-hand twist or left-hand twist.
The above aspects of forensic bullet analysis are all well and good, but for any of the examination to be done, the bullet itself must first be recovered. The recovery of a bullet poses a fundamental paradox for a forensic firearms examiner – how can an intact bullet be recovered from a crime scene that has only the markings from the barrel, and nothing else? The answer is that it cannot. In order for a bullet to stop, it must hit something. At the scene of a shooting, that might be an object or a person. Depending on the circumstances of the shooting, the bullet might have bounced off other objects along the way, and therefore ricocheted into the final target. If the bullet can be recovered, then, it not only will contain the markings from the land and grooves of the gun, but also will be deformed by whatever it hit, scraped against, or bounced off. It is the job of the forensic examiner to look past the deformity of the bullet and establish the original markings to the best of their ability. This can be difficult. The best place usually to recover intact bullets at a crime scene is, unfortunately, a body.
Other marking on bullets have also been compared time to time. Skid marks are marks parallel to the axis of the bullet made when the bullet initially enters the rifling; the edges and surfaces of the lands will scrape along the bullet surface before the bullet is fully gripped by the rifling. The term “skid mark” is also applied to marks near the nose of a bullet caused by contact with the forcing cone in the barrel of a revolver. The forcing cone is a flare at the breech end of a revolver barrel that is intended to guide the bullet into the rifling. Skid marks made by the forcing cone of a revolver are hard to reproduce; a number of bullets might have to be test fired before one strikes the same spot on the forcing cone as the questioned bullet. Slippage marks are made on a bullet when it slips along the tops of the lands without being gripped by the rifling. Slippage marks are the result of the barrel being worn or having been bored out. Slippage marks may also result if a sub-caliber bullet is fired in a weapon. Slippage marks are hard to replicate in test firings.
A variety of markings on cartridges can be compared microscopically. There is a logical hierarchy to follow in conducting such comparisons. Firing pin impressions and breechblock markings (also called bolt-face signatures) should be compared first because they can only be produced by firing a cartridge in firearms. Chambering, extractor, ejector, and magazine marks may be made by loading a cartridge in a weapon without firing it.

References:

Axelrod, A., & Antinozzi, G. (2003). Criminal Investigation. Indianapolis, USA: Alpha Books.
Bachrach, B. (2002). Development of a 3D-based automated firearms evidence comparison system. Journal of Forensic Sciences, 47(6), 1253–1264.
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