Cartridge Case Matches
The Firearms Unit examines firearms for operability, compares fired evidence bullets and cartridge cases to test fired ammunition, and makes determinations of firing distance.
The general examination of the firearm includes determination of loaded or unloaded status, rendering the firearm safe for handling and examination, and a determination of operability.
Fired bullets and cartridge cases are examined with the aid of microscopes to determine what type of firearm may have been used. Striations (scratch marks) on fired bullets are unique to each firearm, as they are caused by imperfections in the barrel. Based on a finding of sufficient similarity between the questioned and test fired bullet, the Criminalist can reach the positive conclusion "this firearm fired this bullet." Similarly, fired cartridge cases can be identified to the firearm in which they were fired, through microscopic comparison of marks left on the cartridge case by the firing pin, chamber, extractor, ejector or breech face of the firearm.
Firing distances are determined by examination of target surfaces for gunpowder, lead, and nitrites. The pattern of these depositions gives information as to the probable firing distance.
Trajectories of bullets are determined to aid in the reconstruction of events at a shooting scene.
Obliterated serial numbers on firearms may be restored through special chemical processing the obliterated surface.