Forensic firearms examinations • identification of a bullet, cartridge case, or other ammunition component as having been fired by or in a particular firearm. • the possibility for such singular identification can be attributed to specific machining processes used in the manufacture of firearms.
Rifling • Helical grooves (rifling), cut into the bore of a barrel of a firearm during production, increase the accuracy of that firearm • Effects: • Spin • Stabilized • Marks on the bullet (lands and grooves)
A rifled barrel: • will contain scratches, scrapes, and other minute nicks and flaws • exacerbated through the subsequent use and discharge of the firearm • or as a result of natural wearing processes such as rusting and corrosion • bullet will have unique microscopic markings as a result
Firing pin impressions: • During the discharge of a firearm, the firing pin strikes the primer of a cartridge, creating microscopic contact marks and unique indentations.
Breechface impressions: • As the powder within the cartridge begins burning, the cartridge case is propelled backwards against the breechface with enough force to be impressed with the characteristic microscopic features of that surface.
When no firearm is recovered: • rifling parameters taken from the recovered bullets can be used to determine the possible manufacturers of the firearm from which the bullets were fired. • General rifling characteristics or GRC are the identified rifling pattern (i.e. 8/right) and the diameters of the individual lands and grooves. • Using these parameters, firearm examiners can search through a database of known rifling data.
A typical GRC search: • a 9mm LUGER bullet, fired from a 6/right rifled barrel, with a land width of .055 and a groove width of .125.
FBI Database: • The database used by firearm examiners all over the world is compiled and maintained by the FBI. • The FBI has been recording rifling data from firearms for over 20 years. • As new firearms are manufactured, the rifling data is included in the database.
Case Study • Many years ago, a double murder occurred in our community. An elderly couple was found shot to death in their home by their son. • It appeared from the scene that there had been a break-in but the son was immediately a suspect. Investigators had a feeling that the son may have committed the crime but could not determine if he had owned or had access to a firearm. • Bullets from the scene were examined and it was determined that they were of 38 caliber and were fired from an 8/right rifled barrel.
Case Study • A list of manufacturers was compiled and given to the investigators. • Using this list, the investigators searched a weekly "Bargain Mart" classified ad publication. By calling the ads for firearms included on the list of possibilities, the investigators found one individual who said he had sold his revolver to an individual who matched the description of the son. • The son was then picked out of a police line-up by the individual who had sold him the gun. When confronted with the evidence the son confessed to the killings.