Forensics officers retrieve a replica gun from the garbage in an area where three people were shot in Toronto, Canada, in 2006.Photo Tannis Toohey/Toronto Star/Zuma Press
Firearm examiner Ed Wallace fires a shot from a handgun into a water tank in the Bexar County Criminal Investigation Laboratory in San Antonio, Texas.Photo R.McLeroy/San Antonio Express/Zuma
FEATURED IN INVESTIGATION
Criminal investigation requires extensive knowledge of the evidentiary value of the physical evidence associated with firearms. In the vast majority of homicide cases, the victim is killed with a gun. According to United States Department of Justice statistics, 55 percent of homicides were committed with handguns and 16 percent with other types of guns shotguns and rifles in 2005. This article discusses the science of firearm evidence and what logical inferences investigators may make from an examination of the use of a firearm at the scene of a crime.
Firearms, Bullets & Associate Evidence
Because of the evidentiary value of locating a firearm used in a crime, investigators go to great lengths to find the weapon, searching storm drains, mailboxes, abandoned vehicles, lakes, rivers, etc., sometimes in a several-mile radius. A variety of physical evidence can be gathered from firearms, bullets, shot pellets, slugs, shell casings and gunshot residue. There may be fingerprints, blood or other biological evidence on the firearm, clips, magazine or bullets. The serial number on a firearm may prove ownership and/or provide an investigator with the ability to trace a gun from its manufacturer to sale and ownership.
Note several key definitions relating to firearms. The diameter of the interior of the firearms barrel is called the bore. Shotguns are smooth-bore weapons while rifles and pistols have rifling. The caliber is the diameter of a bullet. Because the bullet s caliber is slightly larger than the bore, the rifling grips the bullet as it travels through the barrel, causing the bullet to rotate. In the firearm barrel s interior, the high sides are called lands and the low sides are called grooves.
The bullet's rotation through a rifled barrel causes striation on the bullet. When a spent bullet is located at a crime scene or taken from a victim s body, investigators can use a comparison microscope to compare the marks on a bullet with those test fired in the laboratory through the gun. In cases where a bullet but not a firearm is recovered, the class characteristics of a fired bullet yield information about the weapon the bullet was fired from, such as a range of makes and models that suggests the type of weapon. More often than not you ll recover only bullet fragments from a crime scene because bullets are damaged by impact with objects such as walls, floors and human bones.
Shotgun pellets do not lend themselves to positive identification as to whether or not they were fired from a particular shotgun. The diameter of the shotgun barrel is called the gauge. The size and shape of a recovered wad (a paper or plastic wad that pushes the pellets through the barrel of the shotgun when fired) can often determine the gauge of the shotgun and/or manufacturer.
Cartridge & Shell Casings
When it strikes the cartridge case to fire the bullet, the firing pin leaves an impression in the soft metal, which you can also examine under a comparison microscope. On a center-fire type cartridge, the firing pin must strike the center of the cartridge to ignite the powder. On a rim-fire cartridge the firing pin can strike anywhere around the base of the cartridge to ignite the powder.
The slide action of a semi-automatic or automatic firearm may contain marks made when the casing is pulled out of the chamber, or ejector marks made when a cartridge is forced out of the weapon. And because someone loaded the weapon, it s always possible you can locate fingerprints on a cartridge case.
The advent of computer technology has allowed for the storage of bullet and cartridge-case characteristics similar to the Automated Fingerprint Identification System (AFIS) used to store fingerprints. The Integrated Ballistic Information System (IBIS) developed for use by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) provides microscopic images of identifying characteristics found on expended bullets and cartridge casings. In 1999, the FBI and ATF joined together to create the National Integrated Ballistics Information Network (NIBIN; www.nibin.gov), which provides guidelines, assistance and equipment to federal, state and local laboratories to house an automatic search system to produce databases of files from bullets and cartridge casings found at crime scenes or test fired from recovered weapons located at crime scenes.
When you repeatedly fire your duty weapon during range practice, you ll notice gunpowder residue visible on your hand. This is because when a gun is fired, the bullet is propelled forward by the explosion of gases created by the ignition of the powder in the cartridge. However, all of the powder is never expended, and partially burned particles of gunpowder and smoke are propelled out of the barrel. Although you may not be able to see it, powder is also blown out laterally on revolvers and during the ejection of the cartridge case when an automatic pistol is fired. Objects in close range of the barrel, such as the hands firing the gun, may receive the residue of gunpowder. This most commonly takes the form of nitrates, barium and antimony from the primer in the cartridge, which is a byproduct of the incomplete burning of powder.
Powder traces on a person s hands or clothing is powerful but not conclusive evidence they fired a gun. A neutron activation analysis (NAA) test on a person s hands will detect the metal residue (barium and antimony) present when a gun is fired. Although wiping or washing the hands with soap substantially reduces residue levels, the sensitivity of an NAA test is so great that it readily detects even minute traces. The sample must be taken within a few hours of the shooting and is obtained with a cotton swab saturated with diluted nitric acid.