Ballistics: The Science of Guns
To develop the science of firearms comparison, the most important tool was the microscope. The earliest crude microscopes were invented in the 1600s, allowing a magnification of ten to twenty times, but images were still blurred. The invention of the compound microscope that relied on multiple lenses fused together improved the situation, as optics magnification and clarity increased exponentially.
From years of data collection and experiments, firearms identification specialists can:
- Compare bullets and match them to a specific firearm
- Accurately estimate the distance of a shooting
- Detect gunpowder residue around wounds and on shooters
- Restore obliterated serial numbers
The following is based on information from numerous crime scene investigation and evidence manuals, a tour of two crime lab firearms sections, as well as books like Bodies of Evidence, Hidden Evidence, and Practical Homicide Investigation.
Guns and ammo
Briefly, firearms these days are of two basic types: hand-held and shoulder. Of hand-held pistols, there are single-shot or multiple-shot, like revolvers and self-loading pistols. Shoulder firearms have long barrels and include rifles, machine repeaters, and smoothbore shotguns. There are many variations on these basic types.
In mass-produced guns, different makes and models have standardizing characteristics. Since the 18th century, guns have been made with internal helical grooves cut into the barrels that are similar to the threads of a screw. They form "lands," or metal ridges between the grooves. The lands grip the bullet and give it accuracy, range and spin.
The interior part of a gun barrel is the bore, and the caliber of a bullet is determined by the bore's diameter, expressed in hundredths of an inch or in millimeters. As already noted above, when a bullet travels through a gun barrel, the bullet's metal gets worn in a unique pattern by the harder metal of the barrel. Any bullet fired from a specific gun will show the same marks, unless there's been some intentional alteration between firings.
"Rifled" weapons (rifles and many handguns) fire single bullets, and the weapon may also eject shell casings. If no casings are found at the scene, it may indicate that the shooter used a revolver, which retains spent cartridges until manually reloaded. Smoothbore shotguns fire multiple pellets.
Bullets found at the scene (or in a victim) offer plenty of information, and investigators are looking for two specific parts:
- The bullet (lead or lead alloy, and may be jacketed in another metal)
- The compartment containing the propellant (black or smokeless powder)
- The cartridge casing that wraps around all of this and is stamped with manufacturer's mark and caliber
- The soft metal cap at the cartridge head containing the primer
When triggered, the gun's firing pin hits the cartridge in a place that has a shock sensitive primer, or explosive. That charge sets the gunpowder into a rapid burn, which builds pressure until the cartridge can't contain it. This forces the bullet outward and the cartridge backward against the weapon's breech. The impact stamps a distinct impression onto the cartridge head. (There's a little give in each gun, so the stamp isn't necessarily always in exactly the same place.) In addition, the mechanisms that extract and eject the shell leave their own characteristic marks. Whatever scratches the cartridge picks up are unique to that gun.
Matching an ejected casing to a gun may mean shooting the suspect gun (if recovered) in the lab's firing range. Then a comparison can be done between the casing from the scene and the one shot by the scientist, as they did in the Sacco and Vanzetti case. People sometimes mix different brands of ammunition, so it's necessary to use the brand under investigation. Since the test bullet must be recovered, the gun is fired either into a tank of water for very soft metals or into thick cotton batting for others. Then it's compared for microscopic scratches.
On a comparison microscope, the views are linked optically. It takes skill and experience to make a definitive match, but it's possible to say that a certain bullet came from a certain gun, and only that gun.
If the gun is not recovered, there's yet another approach.
It's possible to tell something about the make of a gun from the type of cartridge case or bullet found. The direction of twist refers to the way the rifling gives a right or left-handed spin to the bullet when fired. Smith & Wesson guns have five lands that twist to the right, for example, and a Colt .32-caliber revolver has six that twist to the left. To get this determination, the analysts point the casing away and examine how the lines of striation angle from base to nose, and they add up the number of marks around it. To say that two bullets are from the same gun, the land impressions must match both in number and on the angle of twist.
These days, crime labs can use a computer analysis to make such comparisons. Computers are networked to statewide and national databases (even international), similar to the system for fingerprints. One is called Drugfire, sponsored by the FBI. The Bureau of Alcohol, Tobacco and Firearms has a program called Bulletproof for bullet images and Brasscatcher for cartridge cases. IBIS, by Forensic Technology, also offers automated comparisons of evidence images.
For example, when there's a shooting in one locality and the casing is recovered, it's put into the Drugfire database. (If the gun is recovered, it may be test fired and that spent casing used.) To get a comparison on the way a mark is left by the firing pin of that gun, the ejected case is placed in a device with a video camera that links to a computer. An image of the marks is run through the database to get the closest matches. If matching the firing pin's mark, an image of the base is positioned on the screen and the computer then lines up the other images for comparison, 24 at a time. They can be moved around to get better side-by-side views. Examiners eliminate all but the most likely candidates, which go to a microscope for a more precise comparison.
The firing range also has another use. To measure the distance from muzzle to target, the gun is shot from varying distances at a thick cardboard target. The shooter then examines the size of the hole and the diameter of gunpowder residue, because when guns are fired, fragments of unburned powder fly out of the barrel. They don't travel far, only a few feet, but if the gun is close enough for the residue to hit something, it leaves distinct circular patterns, with size depending on distance from target. Thus, a person shot at close range will get what's called stippling, a gunpowder burn. Replicating the size of the burn from different distances yields the actual distance measurement.
Gunshot residue detection
The gunshot residue can also be swabbed from the skin or clothing of the suspected shooter (as long as it didn't involve a gun where the primer chemicals eliminate metals). For example, if a man kills himself with a handheld gun, he will undoubtedly have residue on his hand. It can be subjected to analysis for its composite content under the scanning electron microscope. (Some labs use a different method.) One problem is that just standing near a gun when fired can result in gunshot residue. Yet the experts can still examine relative amounts and make educated guesses.
An interesting aspect of firearm examination is the ability to trace a serial number to a registered owner—-even if it appears to be gone. Although some criminals file it off to prevent a trace, it may still be recoverable.
This stamping process actually goes deeper than the surface numbers indicate, so when criminals can no longer see the number, they believe they've obliterated it. The examiner grinds the metal down past the deepest scrapes to get a strip of polished metal. He or she then applies a solution of copper salts and hydrochloric acid, and that makes the strained area just beneath the stamped number dissolve at a faster rate than the metal around it. That temporarily brings up the number (or a partial), making it available for a photograph before it disappears.
Even beyond firearms analysis and projectile measurements, another aspect of the behavior of bullets gets us into yet another arena: that of the forensic pathologist. Many will attest to the tricky trajectories they have seen when tracing a bullet's path from entrance to exit wound (if there is one). Among the most puzzling cases is that of the bullet trajectories in the body of the late President John F. Kennedy.