Firearms Identification

Firearms identification is the objective, scientific examination and analysis of firearms-related evidence. The scientific role of the firearms examiner in forensic laboratory examinations is multifaceted. In many instances, it is limited only by the ingenuity of the firearms examiner and/or by the evidence submitted to them for examination. Through their expertise, firearms examiners can assist an investigator in a homicide and/or shooting investigation.

Examinations will usually include, but not necessarily be limited to, the following:

• 1.
  Microscopic examination of fired bullets and cartridge cases to determine if they can be identified or eliminated as having been fired from a suspect firearm
• 2.
  Examination of fired “evidence bullets” in an attempt to determine the caliber, make, and type of firearm from which the bullet could have been fired
• 3.
  Administration of a firearms distance determination test
• 4.
  Conducting a fired cartridge case ejection pattern test
• 5.
  Administering a firearms operation performance test
• 6.
  Examination and comparison of toolmark evidence
• 7.
  Conducting crime scene examinations
• 8.
  Execution of shooting scene reconstructions

The principles of firearms and toolmark identification are based upon the theory that it is impossible to manufacture two items that will be exactly alike. Although items may appear similar to the naked eye, when examined under a microscope by a trained examiner, the imperfections and differences are readily visible.

In some cases, only a fired bullet or fired cartridge case is submitted to the crime laboratory for examination, because no firearm has been recovered. The firearms examiner will microscopically examine the “evidence item” in an attempt to determine the caliber, type, and manufacturer of the firearm. If a firearm is being examined, the identification also includes the examination of the firearm to determine whether it operates safely, whether it has been altered, whether the trigger pull has been lightened, whether the safeties operate properly, and whether the firearm has been fired. The examination will reveal the all-around operating condition and functioning of the firearm.

The manufacturing imperfections caused by the various machining processes during the manufacturing of a firearm or tool are what the firearms examiner will look for when conducting a microscopic examination of the evidence. These imperfections will be displayed on the surface of the machined part as machining marks or “striations” (small grooves or scratches). Microscopic identification is made possible by the manufacturing marks that appear on the surface areas of such items as the interior bore (gun barrel) of a firearm, bolt faces, firing pins, extractors, ejectors, or tools. Through each step of the manufacturing process, microscopic imperfections are left on the surfaces of the manufactured part.