Chapter 18
IN THIS CHAPTER
Getting to know guns
Analyzing bullets and shell casings
Matching rifling patterns
Handling gunshot residues
Guns are a source of pleasure and sport for many people and, when used properly, are safe. But watch a movie, any movie, and odds are that someone, and often several someones, will be shot. Guns have been a movie staple for many years. Old westerns always involved a gunfight or two, and if you view any current TV dramas, you’re likely to see that cops, private investigators, criminals, gang members, and just about everyone has a gun. And real life isn’t far behind. Watch the news or read a newspaper any day of the week, and you’re bound to find that a gun was used in some illegal way.
In fact, guns commonly are used in criminal activities. Besides their obvious capacity for murder and injury, guns are an effective means for gaining control. In armed robberies, abductions, and rapes, having a gun can ensure compliance from the victims. Deaths from gunshots can be accidental, suicidal, or homicidal. In a homicide, evidence from the gun or ammunition often proves to be the perpetrator’s undoing.
This area of forensic investigation is often erroneously called ballistics. In fact, ballistics is the study of how projectiles — bullets, rockets, mortar shells — travel through space. Gun and bullet examination is actually firearms examination, which is performed by specially trained forensic firearms examiners. They commonly have to
Have you ever fired a gun? Most people vividly remember their first time. The sudden explosive discharge is shocking. Even if you expected the recoil, it probably was more of a jolt than you anticipated.
Guns typically are classified as handguns, rifles, or shotguns:
Illustration by Nan Owen
FIGURE 18-1: Anatomy of a handgun.
Rarely do investigators find a smoking gun at a crime scene — in large part because most powders used these days are smokeless — but even a metaphorically smoking gun is hard to come by. Bullets, however, are the next best thing and can go a long way toward helping a forensic firearms examiner determine what kind of gun was used, and by whom.
Unfortunately, an examiner often doesn’t have an intact bullet for analysis. More often, the examiner gets a severely damaged bullet or a bullet fragment; however, even a bullet or fragment that is not severely deformed or too small to use, still can provide a wealth of information.
During the collection and handling of any crime-scene bullets, the investigator must take great care not to damage or alter them. Whether removed from a body in surgery or during an autopsy or from a wall at the crime scene, bullets need to be handled carefully. For example, a bullet can be damaged when grasped with a surgical instrument or pried from a doorjamb, altering the striation pattern and making a match of the bullet with a suspect weapon impossible.
Bullets also may have important trace evidence attached. Paint, fibers, and other materials may cling to the bullet as it passes through or ricochets off walls, doors, bricks, or window screens. Sometimes, a fired bullet yields DNA, if investigators find small bits of flesh and blood attached to it.
A forensic firearms examiner assesses the chemical and physical composition of a bullet to determine its manufacturer and shorten the list of suspected weapons. You can find most bullet types in weapons of varying calibers and muzzle velocities, but softer bullets are more common in low-velocity weapons, and harder or jacketed bullets usually are used in high-velocity weapons.
Bullets are categorized as follows:
The caliber and type of bullet are important in determining what weapon was used in a crime. If the bullet is intact, or mostly so, the police can determine the caliber by simple measurement. Severely deformed bullets can be weighed, but weighing them doesn’t give the exact caliber; it serves only to eliminate some calibers. For example, the weights of .22 caliber and .44 caliber bullets vary considerably. Investigators can often easily determine whether the bullet is jacketed and, if so, with what, thus helping to identify the type of bullet and ultimately the type of weapon.
Shotgun pellets can be extracted from the victim during surgery or an autopsy, and from walls, furniture, and other surfaces. The size of the shot doesn’t reveal the gauge of the shotgun but does provide information about the ammunition used. The various sizes of shot are numbered; the lower the number, the larger the pellets within the shotgun shell. For example, Number 8 shot is smaller than Number 4. Pellets vary in size from 0.05 inches for Number 12 shot to 0.33 inches for 00 shot, the largest, which also is called double-O buckshot. An examiner measures the diameter of any recovered shot and estimates what size shot was used.
A shell casing is the portion of the cartridge that remains after the powder explodes and the bullet is gone. Shell casings often are the only evidence a firearms examiner has to work with. Fortunately, they retain many marks that are of interest to the examiner, including:
Bullets don’t pass through gun barrels unscathed but are nicked and scratched along the way. These markings help forensic firearms examiners match bullet to gun type, and maybe even to one particular gun.
A spinning bullet is a more accurate bullet, so most guns are rifled, meaning spiral grooves are etched or cut into the inside of their barrels to make bullets spin as they’re expelled. Cutting the grooves leaves lands, or high parts, intact between them. The grooves grab the bullet as it travels down the barrel, causing it to spin and thereby greatly increasing its accuracy. Old smoothbore rifles weren’t accurate beyond 100 feet or so, but modern rifled firearms are extremely accurate, some even for thousands of yards.
But the accuracy of spinning bullets isn’t what interests forensic firearms examiners. They’re more interested in how the lands and grooves of the rifling mark the bullet. As I discuss in the next section, these markings provide class and individual characteristics (see Chapter 3).
When a gun barrel is manufactured, the rifling is cut, stamped, molded, or etched inside of it. The depth of the grooves, the width of the lands, and the direction and degree of the twist vary among different types of weapons and different manufacturers. These characteristics help examiners identify the type of weapon that fired a crime-scene bullet and its manufacturer.
The Federal Bureau of Investigation (FBI) maintains a database known as the “General Rifling Characteristics File” to help firearms examiners make these determinations. It lists the land, groove, and twist characteristics of known weapons. Bullet and shell casings likewise can be compared with bullets and casings recovered from other crime scenes that are listed in other databases (see the later section “Searching for answers in databases”).
Because smoothbore weapons like shotguns and older firearms aren’t rifled, their projectiles don’t exhibit any evidence of markings caused by lands, grooves, or twists.
When lands and grooves grab and spin a bullet traveling down the barrel of a gun, they also cut into the bullet, leaving behind characteristic markings that are at the heart of firearm comparisons. These markings, called striations, are linear and parallel to the long axis of the bullet. They’re more prominent on soft lead bullets than they are on metal- or Teflon-jacketed ones. (Check out the earlier section “Breaking down bullets.”)
Silencers are devices that muffle the sound of a gun and can range from a towel wrapped around the barrel to a sophisticated sound-absorbing attachment. These attachments may also leave markings on bullets, but these markings are unpredictable. If the silencer does leave behind markings on the bullet but isn’t available at the time the crime lab test-fires the gun, these marks can complicate the examiner’s attempts to match the bullet to the gun from which it was fired.
As if all these lands, grooves, twists, and striations weren’t enough, each rifled barrel has minute characteristics that differentiate it from all others. A rifling tool cuts through each metal gun barrel a little differently; furthermore, cutting or etching equipment becomes worn and damaged with each use. This progressive wear and tear produces rifling patterns that vary from barrel to barrel. In addition, repeated firing wears down and damages the grooves and lands, adding even more individual characteristics to the barrel and thus to any bullet that travels through it.
Comparing individualizing striations is useful in many situations. The first step in making such a comparison is obtaining an intact bullet fired from the suspect weapon. Most firearm labs have a test-firing chamber. An examiner then views the lab-fired bullet next to the crime-scene bullet, using a comparison microscope (see Chapter 7), which places images of the two bullets side by side to facilitate accurate comparisons.
For example, bullets found at a crime scene can be compared to find out whether they came from only one gun. If not, more than one weapon obviously was used. Likewise, separate bullets, each collected from different crime scenes, can be compared to determine whether the same gun fired them. Such a match strongly connects the two crimes. Most importantly, a bullet removed from a homicide victim can be compared with a bullet that’s been test-fired from a suspect weapon. A match means that you have identified the murder weapon, which, in turn, may be the key to identifying the killer.
Even with only a single bullet or shell casing and no suspect bullet or weapon to compare them with, the firearms examiner often can determine the type and manufacturer of the weapon that fired the bullet. And thanks to computer technology and firearms databases, it is often possible to compare ballistic markings on the crime-scene bullet with the markings on bullets from weapons used in other crimes. The major database is the Integrated Bullet Identification System (IBIS). Maintained by the Bureau of Alcohol, Tobacco, Firearms, and Explosives’ (BATFE) National Integrated Ballistics Information Network (NIBIN), IBIS is by no means complete, but if a bullet or casing from the same weapon is in the system, a match to the crime-scene bullet or casing can connect two or more cases and thus aid in solving them all. This system rapidly compares hundreds of records and indicates any possible matches. An experienced firearms expert then visually conducts the final match.
Without a gun, a bullet, or even a shell casing to work with, firearms examiners may be able to draw some conclusions using chemical residues from expended gunpowder. Traces of these residues can linger at the scene and on the victim and shooter. Although it doesn’t tell the entire story, finding this kind of chemical evidence on a suspect connects that person to the scene and gives investigators a good reason to dig deeper.
As can be seen with bite marks (Chapter 12) and some types of trace evidence (Chapter 17), the chemical analysis of gunshot residue, as well as the chemical analysis of bullets to help determine manufacturer, has drawn fire — no pun intended. Again, the lack of standardization of testing procedures and examiner training has led the FBI and other labs to reconsider this type of evidence.
When a gun is fired, the primer and the powder explode within the cartridge, forcing the bullet down the barrel. Much but not all of the explosive gases and particulate matter produced by the explosion follows the bullet (see Figure 18-2). However, some of these materials escape through openings in the weapon. This factor is particularly true of revolvers, which tend to leak more gases than other types of weapons.
Illustration by Nan Owen
FIGURE 18-2: Residue from exploding primer and powder when a gun is shot.
Wind and rain can change the pattern of or lessen the spread of the GSR cloud. As a result, an examiner may find GSR in unexpected areas. On light-colored clothing, GSR patterns are readily visible as smudges or smears, but on dark, multicolored, or bloodstained clothing, the patterns can be obscured.
The goal of GSR analysis is placing a suspect near the gun when it was discharged. Unfortunately, simply being near the gun when it discharges or handling the gun afterward can leave behind GSR on an innocent person. GSR also tends to fade rapidly and usually dissipates after a few hours. It can be wiped or washed away, which is why testing needs to be accomplished as soon as possible after the gun is fired.
Investigators inspect any suspect’s hands, face, and clothing and obtain samples. The old paraffin test, where melted paraffin is used to pick up residues from the shooter’s hands, rarely is used anymore. Instead, investigators swab the suspect’s hands, arms, and clothing with a moist swab or filter paper to obtain samples.
In Chapter 12, I tell you how the distance between the muzzle of the gun and the victim affects the anatomy of entry wounds. When a gun is fired, hot gases and burned and unburned gunpowder particles follow the bullet out of the barrel. The ME usually can estimate the firing distance by the effects of these substances on the wound.
Sometimes, however, the victim is shot through clothing, such as a shirt or jacket, and the tattooing and charring that the gases and unburned powder normally cause on the victim’s skin are intercepted by the clothing. The ME therefore is left with few, if any, distance markers on the victim’s skin. Enter the firearms examiner.
The examiner uses the suspect weapon and similar ammunition to perform a series of test firings into similar fabric and accurately estimate the distance. Test firings are made at 6 inches, 1 foot, 18 inches, 2 feet, and 3 feet, and the examiner then compares the residue patterns on the test garments with those found on the victim’s clothing to find the closest match.