AFTE Store - Primer Cup Properties & How they Affect Identification

Firearm examiners encounter many different cartridge and gun types while performing casework, and ammunition and firearm combinations can ultimately impact identification. A study was conducted to evaluate the properties of primer cups and determine how they affect the quality and quantity of breech face marks. Seven different types of 9mm Luger cartridges and four different firearms were used with four different breech face manufacturer marks: parallel, circular, arc, and granular. Hardness and primer thickness were determined before firing. Hardness was also determined after firing. Nickel plating added to the hardness of primer cups, and hardness significantly increased after the firing process, especially at the location of the firing pin impression. Fired cartridge cases were then compared to a breech face cast of the firearm they were discharged from. The results of these studies demonstrated notable and consistent differences in marks on the primer cup. It was also demonstrated that cartridges with similar initial physical properties often obtained marks in a similar pattern.