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Like bullets, cartridge cases can be identified as having been fired by a specific firearm. As soon as cartridges are loaded into a firearm the potential for the transfer of unique tool marks exists. However, the cartridge does not have to be fired for these marks to be transferred. Simply loading a cartridge into a firearm can cause unique identifiable marks that can be later identified.
Cartridge cases are mostly made of brass but can also be made of other materials such as steel and plastic. Cartridge cases come in a variety of finishes but all are made of a material that is softer than the materials found in a firearm. Any surface of the cartridge case that meets the inner workings of the firearm may be marked.
Tool marks produced on the cartridge cases will be in two basic forms. As the microscopic striations found on bullets, cartridge cases can pick up striated action marks. These “scratches” are produced when the cartridge case moves laterally against the tool (inner surface of the firearm) producing a scrape or striated mark. The other form of marks that can be left on a cartridge case are impressed action marks. Impressed marks are created on cartridge cases when it impacts the tool (again, the firearm) with adequate velocity or pressure to leave an impressed or indented mark.
Cartridge cases are compared to fired standards from a firearm using a comparison microscope. Standards are first examined to determine what marks, if any, the firearm is consistently reproducing. Evidence cartridge cases are then directly compared to the standards to see if they too are also similarly marked.