Report of the President's Commission on the Assassination of President John F. Kennedy

General Principles of Firearms Identification

This section explains the general principles underlying firearms identification. A cartridge comprises a primer, cartridge case, powder, and bullet; pulling the trigger causes the firing pin to detonate the primer, igniting the powder and propelling the bullet through the rifled barrel. Rifling imparts a stabilizing spin, and weapons of the same make and model share rifling characteristics (groove count, land count, and twist direction), all of which are engraved onto fired bullets. Beyond rifling, every weapon bears distinctive, weapon-unique microscopic characteristics on its barrel, firing pin, and breech face, arising from manufacturing processes (machining, grinding, filing), tool wear, and subsequent use, fouling, cleaning, and corrosion. Frazier explained that these accidental marks—scratches, nicks, dimples, and pattern variations—enable an examiner to identify a cartridge case or bullet as having been fired in a particular weapon to the exclusion of all others, as demonstrated by his comparisons of consecutively manufactured bolt faces and barrel pairs, which showed no shared individual microscopic characteristics. Identification is performed by comparing suspect and test-fired items under a comparison microscope, as illustrated by Frazier’s examination of Commission Exhibit No. 543 (a cartridge case from the Texas School Book Depository), where corresponding numbered circles in the photograph highlight matching microscopic marks in the area immediately outside the firing pin impression of the 6.5 mm Mannlicher-Carcano rifle (Commission Exhibit No. 139).