: The hooks and pins that pull the spent casing out and toss it away leave distinct scratches on the rim. The Technology of Tracking

: Microscopic flaws in the manufacturing process or wear and tear from use create a "signature" unique to that specific barrel. 2. Impression Marks (on the casing)

Bullet fingerprinting, more formally known as , is a forensic technique used to match a fired bullet or shell casing to a specific firearm. The "fingerprint" consists of unique, microscopic marks left on the ammunition by the gun's internal components during the firing process. While often portrayed as a flawless investigative tool in television dramas, the reality of its scientific reliability is a subject of significant debate within the forensic community. How It Works: The Mechanical Fingerprint