Firearm identification is a discipline of forensic science whose primary concern is to determine if a bullet, cartridge case or other ammunition component was fired by a particular firearm. During the manufacturing process of a firearm, random imperfections will be left on the breechface, barrel, and other parts of a firearm. The tools used in the manufacturing process are constantly changing as they cut, grind, and shape the firearm. The subsequent use, abuse, corrosion, rust, and cleaning also contribute to the firearm's individuality. Therefore, when a bullet or cartridge case comes into contact with the harder metal of a firearm, unique microscopic marks (imperfections) are imparted onto the bullet or cartridge case. An examiner is then able to compare these microscopic marks and determine whether markings found on the evidence are the same as the markings produced by the submitted firearm. Tool mark identification is similar to firearm identification in that the examiner is attempting to determine if a striated or impressed tool mark was produced by a particular tool. Common instrumentation used by examiners include: comparison macroscopes, stereomicroscopes, balances, calipers, and micrometers. The comparison macroscope allows the examiner a side by side comparison of a fired ammunition component to test shot exemplars (or a tool mark to a particular tool).
Firearms & Toolmarks
Firearms & Tool Marks examiners in the Michigan State Police Forensic Science Division perform a wide variety of examinations, including function testing of firearms, bullet classifications, tool mark examinations, gunshot residue pattern/distance determination analysis, serial number restorations, and at-scene or post-scene shooting reconstruction.
Submitted weapons are test-fired in order to obtain known evidence exemplars which are compared to fired bullets and cartridge cases that are often collected at crime scenes. IBIS BrassTrax 3D and MatchPoint+ are computerized digital imaging systems which capture digital photographs of fired cartridge cases (evidence and test shot exemplars). These images are stored in a database and are electronically compared to one another. An IBIS technician conducts a visual comparison of "high confidence hits" or images that display similarities for possible identification purposes. This system can generate investigative information and connect firearm evidence from previously non-related shooting events
Often firearms are submitted to the laboratory with obliterated serial numbers. The practice of removing the number from a firearm is usually the result of attempting to conceal ownership of a weapon that has been stolen. With the serial number removed, law enforcement officials cannot establish who the legal registered owner is. The examiner can utilize a number of different techniques in an attempt to restore the number, including magnetic particle inspection, chemical etching, electrochemical, and thermal/heat procedures.
Gunshot residue patterns/distance determination analysis can establish how far away the muzzle of a firearm was when it was fired at a victim or object. When a firearm is discharged at close proximity to an object, it deposits what is known as gunshot residue (GSR). GSR is often visible on the clothing of a victim of a shooting incident. This examination consists of a series of microscopic and chemical tests, firing the suspect weapon at predetermined distances, and then a comparison between the evidence and tests to establish an approximate distance in which the firearm was to its target.
Finally, examiners in the Firearms Unit can attempt to reconstruct a shooting scene, which involves measuring angles, examining areas of impact, determining exit and entrance sites, documenting and measuring areas of penetration, and determining the direction from which a shot was fired.