Firing marks left on bullets and cartridge cases can be used to identify the firearm used. There are several types of marks including: 1. Rifling marks on the bullet from the grooves in the barrel. These marks are unique to each gun. 2. Firing pin marks on the primer from the firing pin striking it. Imperfections in the firing pin can be transferred. 3. Breech face marks on the cartridge from the cartridge striking the breech face on firing. Imperfections are imprinted. 4. Extractor and ejector marks on the cartridge case from the mechanisms removing the spent case from the firearm.