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.
An NAA test is important for investigating cases of suspected suicide with a firearm. (It s critical to place plastic bags on the hands of the deceased until the NAA test can be performed.) The test is also useful when suspects claim they didn t fire a gun or when a shot is fired and a gun is located among several suspects who all deny firing the weapon.
Contact shots are made when the muzzle of the firearm is pressed against the body, causing gunpowder, metallic particles and often bits of the person s clothing to be driven inward. Contact wounds may result in the shape of the muzzle of the weapon causing an impression in the skin.
The entry wound caused by contact with a firearm is distinctive. The weapon s explosion forces gases into the wound and then back out causing a bursting effect that often looks star-shaped with tissue directed outward. Objects at a distance from the muzzle of a firearm receive little or no gunpowder residue. When a person is shot at close range (but not in physical contact with the firearm), tattooing appears on the skin in the area where the bullet entered the body. This distribution of gunpowder particles around the entry wound allows the investigator to assess the distance the firearm s muzzle was from the victim.
The powder-residue pattern on the skin will vary depending on the type of weapon and ammunition used, so the accuracy of determining the distance the weapon was from the victim depends on whether a firearm is recovered and the type of ammunition. However, in most cases, if the muzzle of the firearm was an inch or less from the victim when it was discharged, a large concentration of gunpowder particles will be observed around the bullet s entrance wound. From 12 18 inches, the discharged firearm s muzzle will produce a wider pattern of gunpowder residue. In most cases, firearm wounds inflicted from 36 inches or more will not deposit noticeable gunpowder residue. The absence or presence of gunpowder residue may prove important in determining whether a case is a suicide or murder.
Marking Firearm Evidence
Follow two rules for marking firearm evidence. First, make the weapon safe. Second, never place a mark directly on any type of firearm evidence. Wear gloves. Pick up the firearm by the knurled grips or by the edge of the trigger guard. Place the firearm in a properly marked evidence container or affix an evidence tag to the trigger guard.
Bullets, cartridge cases, etc. each go into a separate evidence envelope or bag. With long guns (rifles, shotguns, etc.), affix an evidence tag to the trigger guard. Never insert anything into the barrel of the weapon in order to pick it up. If you find a firearm immersed in water, do not clean or dry it. Place it in a container with the same water completely covering the weapon and transport it to the lab. In all cases, maintain and document the chain of custody. Indicate the weapon s location on your crime-scene sketch and photograph all firearms prior to collecting them as evidence.
Determining a bullet s trajectory helps to reconstruct the crime scene and is helpful in establishing the truthfulness of suspects and witnesses. When a bullet goes through a glass window, both radial and concentric fracture lines in the glass develop. Radial fractures move away from the impact, and concentric lines often form a rough circle. A cone-shaped area usually exists in the glass where the bullet exited. If the cone-shaped area is on the inside, the bullet was most likely fired from the outside, and vice-versa. Take photographs of the window detailing the fractures to the glass. Try determining a bullet s path through glass or other objects by sighting through the hole to trace the path back to the location where the bullet was fired from if the path of the bullet wasn t altered by striking another object. At night, shine a flashlight through the bullet hole to trace the bullet s path.
At around 1900 hrs my partner and I were called to the residence of a single family home in Hartford, Conn., on a report by initially arriving officers of an apparent suicide. Officers led us to a bedroom where a 35-year-old male was observed lying face down on the floor with a single gunshot wound to his right temple. A .38-caliber revolver was on the floor next to the body within inches of his right hand. The deceased was dressed in a business suit.
The deceased's wife was sobbing in the living room. She had informed the first-responding officer that she came home from work and found her husband had shot himself. Further, she said her husband had been despondent for some time after being passed over for a promotion at a local insurance company. The wife informed me she was a nurse and other than taking her husband s pulse, she hadn t moved or touched anything in the room.
Examination of the bullet wound to the right temple revealed no tattooing on the surrounding skin. We recovered the bullet in the wall. Its trajectory was not consistent with a positioning of the body plausible for a suicide. Examination of the weapon revealed one shot was fired. The revolver s cylinder held an empty cartridge case and five live bullets. Postmortem lividity stains on the victim s body indicated he d been turned over after death.
The victim s hands were bagged and an NAA test performed at the medical examiner s office. The wife was given her Miranda warnings, and when presented with the evidence in the case and asked if she would submit to an NAA test on her hands, she confessed to shooting her husband following an argument about an affair he was having.
Sometimes murders get solved easily most of the time they don't.
The above material is a basic primer for investigating firearm evidence. For more advanced investigative techniques, I recommend the reference book Criminal Investigation: A Method for Reconstructing the Past by James W. Osterburg and Richard H. Ward (Anderson Publishing, 5th ed., 2007). This reference book should be part of every investigator's library.
Portions of this article were taken from the author's forthcoming book on criminal investigation, The Path of the Hunter.