CHAPTER 8 FIREARMS AND TOOLMARKS
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Chapter 8 Firearms and Toolmarks
This area of forensic expertise is sometimes mistakenly referred to a "ballistics." Ballistics, however, is a separate discipline that analyzes the flight of a moving projectile. There is some ballistics in firearms, but not all firearms are ballistics.
Firearms and toolmarks are often taken together as one "area" of study, though they are markedly different. As a group, this area involves impression evidence. That is, forensic examination of impressions made when one object is impacted by another. In the case of firearms, it examines things like the impression a firing pin makes on a shotgun shell, or the marks made on bullet from the barrel of a weapon.
Historically, evidence involving impressions made by guns onto bullets or casing has been accepted by the courts, even prior to Daubert. Since the decision in Daubert, courts have generally held that such impression evidence meets the Daubert standard. The case of U.S. v. Otero, discussed below, provides a frame of reference on the admissibility of such testimony.
While similar, forensic examination of toolmarks stands on a different legal footing. This type of evidence involves, for example, the marks made by a crowbar used to open a locked cash register. One is hard-pressed to find case law regarding such evidence at all. And if found, it appears any Daubert analysis is limited at best.
Based on this chapter, students will be able to:
1. Identify the class characteristics involved in firearm identification.
2. Identify the sub-class characteristics.
3. Identify the individual characteristics.
4. Have a working knowledge of how these characteristics help connect a particular projectile or casing to a particular weapon.
5. Understand how these characteristics can exclude a weapon from being the source of a projectile or casing.
6. Have a working knowledge of the National Integrated Ballistic Information Network and its operation.
7. Understand the role of gunshot residue.
8. Understand toolmark evidence.
9. Understand the utility and the legal difficulties associated with firearms and toolmark evidence.
One can only imagine the despondency in the Raleigh, North Carolina, community from the spring of 2006 to the fall of 2007. Five people had been brutally murdered. It began on May 12, 2006, when Sam Haj-Hussein, a convenience store clerk, was shot and killed in his store.1 The matter remained unsolved for over a year. Four more murders would occur, all unsolved, until a break in the case.2 Prior to that "break," the North Carolina State Bureau of Investigation had made some progress in the case.3 Bullets and shell casings from each of the murders had been submitted for examination to the state forensic laboratory. Upon examination, it was learned that each bullet and cartridge, all 9mm, was related to the others. Each bore distinctive markings relating it to each of the prior murders. The problem now was finding the weapon, and the person, that fired the bullets that killed the victims.
On November 21, 2007, Raleigh police arrested Samuel Cooper after a bank robbery. During Cooper's attempt to flee from law enforcement, he dropped a 9mm Ruger handgun. That gun was recovered by the police. Further investigation linked that gun to each of the unsolved murders.4 Cooper would confess, and although his confession was challenged for legal reasons,5 the result of the forensic testing played a key role in his conviction.
Forensic study of firearms endeavors to match a particular bullet to a particular gun used in a particular crime. This is done through careful examination of the bullet and casing. Contrary to the portrayal in popular culture, identification of a bullet, and matching it to a particular firearm, is less common than might be expected. Bullets may be severely degraded or destroyed upon impact, and thus not readily available for comparison. Often it is the casing that proves to be the "smoking gun."
Class Characteristics
As with other types of evidence, bullets have characteristics particular to class, as well as to an individual firearm. While class characteristics may at first blush seem less important, these class characteristics can offer valuable clues. While class characteristics will not answer the question of whether a bullet was fired from a particular weapon, class characteristics can eliminate certain weapons from consideration.
Bullets are named by caliber. Caliber refers to the diameter of bullet in inches. Therefore, a .44 caliber bullet is 44/100 inches in diameter. Generally speaking, the caliber of the bullet matches the caliber of the gun. If there is evidence that a .22 caliber gun was used in a crime, the .357 caliber gun that law enforcement seized from a suspect is not likely the weapon used in the crime. If there is evidence that a .44 caliber gun was used in the commission of a crime, law enforcement's successful and proper seizure of a .44 caliber weapon does not mean it was in fact the weapon used. Nonetheless, that seized weapon cannot be ruled out as the one used in the crime. Thus, class characteristics such as caliber are important for rule-in/rule-out purposes. Further forensic testing is required to identify individual characteristics that would tend to prove whether this particular bullet was fired from this particular gun. In this respect, bullet caliber is similar to blood type. If a perpetrator's blood is left at the scene, it can be examined for type, and thus rule out everyone with a different type of blood and make suspect anyone with that blood type. Further forensic examination of the DNA can then locate individual characteristics to pinpoint a particular person.
Similarly, shotguns may likewise be identified by a particular class characteristic. This identifier is the "gauge" of the shotgun. Unlike rifles or handguns that discharge a single bullet when fired, shotguns propel a number of small round metal pellets, referred to as "shot." Gauge refers to the number of metal pellets in one pound of shot. The smaller the gauge of the shotgun, the larger the shot. Thus the pellets in a 12-gauge shotgun are larger than the pellets in a 20-gauge shotgun. The gauge of the shotgun shell matches the gauge of the shotgun.
Sub-Class Characteristics
There is a class of markings known as sub-class markings. Class characteristics are those that result from the manufacturing process. They are markings impressed from the manufacturing process. For example, as discussed below, a manufacturer will put grooves in a weapon's barrel that cause the bullet to spin. This leaves a distinctive mark on the bullet. The left- or right- direction of the groove is a class characteristic. Generally, it will be common to all guns of that type made by that particularly manufacturer.
By contrast, sub-class marks are unintended. In the manufacturing process, metals fatigue. The blades used to cut grooves dull. This leaves other marks, or perhaps has an effect on intended marks. But as a result of the manufacturing process, other marks are left on only some weapons. While the class characteristics remain, distinctive marks of a sub-class can appear.6
Individual Characteristics
In the initial investigation, general class characteristics allow law enforcement to focus on a particular type of weapon. Further forensic testing can link a weapon to the bullet fired from it. By examining the particular marks made in a bullet as it travels down the barrel and exits the muzzle of the weapon, an examiner can determine with some certainty whether a particular bullet was fired from a particular gun. Similarly, by examining the imprints on a bullet or shell casing an examiner can make a similar determination.
Characteristics Based on Wear
These types of markings are also unintended. They arise perhaps over a short period of time. They can arise and appear because of use and abuse sustained by the weapon. Therefore, two casings or bullets, fired a lengthy period apart, may not have the same characteristics, although they were fired from the same weapon.
Bullets
Vagaries in the manufacturing process makes every firearm unique. Generally, however, a firearm manufacturer will place a spiral groove along the length of the barrel. Some manufacturers groove the barrel clockwise, others counterclockwise. The spiral grooves spin the bullet as it is ejected from the muzzle in order to increase stability and accuracy in flight. The process of imprinting spiral grooves is called "rifling." "Grooves" refer to the spiral cuts in a gun's barrel. "Lands" refers to the ridges between the grooves. The lands and grooves leave a distinctive pattern on a fired bullet. These marks are collectively referred to as "striations." They are individual characteristics, particular to a weapon.
If law enforcement is able to obtain a reasonably intact bullet, perhaps after an autopsy of a victim, that bullet can be compared against one test fired from the weapon thought to be used in the crime. The forensic firearm technician will fire a bullet from the potential weapon into a tank of water or gel. In this manner, the bullet maintains its essential characteristics and is not destroyed. The test bullet is then retrieved and compared to the bullet used in the crime by means of a comparison microscope. In so doing, the forensic examiner can now testify in court regarding the methods of testing and the results. Again, because of the nature of the manufacturing process, bullets from different guns will be very different from one another, while bullets fired from the same gun will have identical, or nearly identical, striations.
Figure 8.1 Comparing striations in a subject bullet to a test-fired bullet in a Leeds Forensic Systems LCF3 comparison microscope. image courtesy of Leeds forensic systems.
Casings
Like bullets, a bullet's casing or shell can be examined for individual identifying characteristics. Examining casings...
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