When a semi-automatic firearm fires, it ejects the empty cartridge case — and that small piece of brass is often more informative than the bullet itself. In the split second it spends in the weapon, the gun stamps several marks onto it.
The marks a gun leaves
Each case can carry:
- a firing pin impression, where the pin struck the primer to fire the round,
- breech-face marks, pressed into the case head as it slammed backward against the breech,
- extractor and ejector marks, from the mechanism that pulled and threw the case clear.
Together these form a signature that can be compared, under a comparison microscope, to a case test-fired from a suspect weapon.
Linking cases and scenes
Because the same gun leaves consistent marks, cartridge cases can:
- link a case to a specific firearm, and
- link shootings to each other — the same gun used across multiple scenes.
National systems such as NIBIN store images of these marks so investigators can find such links even before a weapon is recovered.
Reading the obvious, too
Beyond microscopic marks, a case tells you its calibre and carries a headstamp identifying the manufacturer and often the calibre — useful investigative detail on its own. As with all firearms identification, examiners increasingly express their conclusions with appropriate, well-defined confidence rather than absolute certainty.