Analysis of Firearms and Ballistics Evidence

Firearm Identification

Forensic firearms identification is mainly concerned with determining whether a bullet or cartridge was fired by a particular weapon. To do this, a firearms expert will first evaluate the caliber of any bullets or casings found at the crime scene, as well as the suspected weapon if it is in custody. Next, the casings will be inspected using a comparison microscope for any unique markings caused by the specific firing mechanisms within the gun and the way in which the gun was manufactured. Gun barrels are made by hollowing out a solid bar of steel with a drill. This process results in microscopic drill marks that are random and irregular. No two rifled barrels, even those manufactured in succession, will have identical rifling impressions. The rifling impressions contain microscopic striation markings, or scratches, on the surface of the casing of the bullet. These subtle differences in the markings allow investigators to compare a bullet from a crime scene to a bullet that has been test fired from the weapon they suspect was used in the crime. Because these microscopic scratches are unique to a particular weapon, they are usually considered individual evidence. If the unique irregularities of the striations match, a link has been made to that particular weapon, and subsequently a possible suspect.

Casings can also be evaluated based on the impressions left by firing pins. While the markings left by firing pins are considered class evidence, matching firing pin marks can add support to any rifling impressions found.

Usually recovered bullets only yield a small area with intact markings. Grit, rust, and mutilation upon impact make it rare to find a perfect match all the way around a bullet. When collecting lodged bullets, investigators must be very careful to prevent additional scratches or mutilation.

In order to test a suspected weapon, investigators obtain bullets matching the same type and caliber from the crime scene and load them into the suspected weapon. The weapon is then test-fired in a special water chamber. The test-fired bullets are collected and evaluated for characteristic markings. Comparisons can then be made between the evidence bullets and the test-fired bullets to see if there are any matches.

In addition to bullet evidence, there may be other types of evidence found on a gun such as blood, fingerprints, or other trace evidence. Investigators must carefully examine the firearm for these other types of evidence which may be even more telling that the striations on a bullet! For example, finding the victim's blood (confirmed by DNA analysis) on a gun in possession of the suspect would be individual evidence.

Gunshot Residue

The firing of a weapon not only propels residues of gunpowder and other components toward the target, but also back toward the shooter resulting in trace deposits on the firing hand, nearby surfaces, and clothing. Thus, gunshot residue can provide very important evidence. Gunshot residue consists of burned and unburned gunpowder, vaporized, and particulate lead, and primer residues of lead, barium, and antimony. In an investigation, it is often necessary to determine the distance between the firearm and the target, as well as the distribution of powder patterns or the spread of shot pattern. Investigators make careful measurements and documentation of all visible gunshot residues to determine this information. The Griess Test is a chemical test that is used to develop patterns of gunshot residues around bullet holes on clothing or other fabric. It works by detecting nitrites in gunshot residue. Photo paper is placed over a bullet hole and heated with an iron containing a weak vinegar solution. The vinegar solution is applied to the area of the bullet hole using the steam feature on the iron. The vinegar reacts with the photo paper to make the nitrites visible as an orange color pattern on the photo paper. Because this method does not destroy the item of evidence, it is usually conducted first followed by other subsequent testing. A Sodium Rhodizonate test may be conducted to detect any lead residues in the areas of suspected gunshot residue. It is conducted by spraying the area of suspected gunshot residue with a solution of Sodium Rhodizonate. If lead residues are present, the sprayed area will turn a bright pink color on the item. To confirm the presence of lead, the bright pink areas are then covered with diluted hydrochloric acid. If the area turns blue in the presence of hydrochloric acid, the presence of lead is confirmed.

Once all testing is completed to locate and confirm gunshot residue, the investigators will then test the weapon with the same type of bullets at various distances to come up with a minimum and maximum distance required to reproduce the same general gunshot residue pattern. Shots are fired at various distances into targets holding white cotton and denim fabric. These cloth targets are inspected for gunshot residue patterns consistent with the evidence collected and can then be used as an approximation for the distance in which the gun was fired from the victim or object.

Ballistics

A very important part of firearms evidence investigations is ballistics. Ballistics is the study of what happens to moving projectiles in the barrel and in flight; their trajectory, force, impact, and penetration. The trajectory, or the path a bullet travels from muzzle to impact, of a projectile can be determined if two reference points for the projectile can be found. Here is an example of trajectory comparison:

Trajectory comparison on graph of different guns and the trajectory is compared in yards

Reference points can be bullet holes in objects or victims such as an entry point and exit point on a victim. A bullet does not travel in a straight line over a long range, but rather in an arc; this is due to the force of gravity on the bullet. Investigators can use lasers to trace a straight-line path between the reference points to help determine the position of the shooter based on the idea that the shooter would have discharged the firearm somewhere along that line.

Collection and Preservation of Firearm Evidence

The most important consideration for collection of firearm evidence is SAFETY!

The best way to pick up a weapon is with 2 fingers on the trigger guard or the checkered portion of the grip which rarely yields prints. If a weapon is found in water, it should remain submerged in a suitable container in the same water to prevent rusting during transport.

When a semiautomatic weapon is used, the discharge pattern of ejected shells should be noted and documented carefully with photography. Carefully collect and pack clothing of gunshot victims to prevent loss of any gunshot residue. If the clothing is wet it should be allowed to air dry out of direct sunlight before packing. Each item should be bagged separately for transport to the lab.

Integrated Ballistic Identification System (IBIS)- is used to associate evidence in previously unsolved crimes. IBIS is a highly technical, computerized image analysis system that records striated images from bullets and cartridge cases and compares them to a national, and growing international, database of images.

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