Along with the identification of fired bullets, the identification of fired cartridge cases and shotshell cases comprise two key aspects of firearms examination. Fired cartridge cases are frequently more useful than fired bullets when linking shooting incidents because they are usually found in greater quantities at crime scenes. They may also bear more microscopic marks of value than recovered bullets, which are subject to mutilation, deformation, and fragmentation.
The identification of fired cartridge cases and shotshell cases dates to 1925 when a comparison microscope was modified to compare the unique microscopic marks produced by firearms on the surfaces of ammunition. This was accomplished by the Bureau of Forensic Ballistics, a private group in New York City led by Calvin H. Goddard.
- cartridge cases and shotshell cases as having been fired in a particular firearm, based on firing pin impressions, breech face marks, and chamber marks,
- fired and unfired cartridge cases and shotshell cases as having been cycled through the action of a particular firearm, based on extractor and ejector marks as well as other mechanism marks.
Although the identification of a cartridge case or shotshell case as having been fired in a particular firearm carries more weight as evidence, the identification of these items as having been at least cycled through the action of a firearm is still significant and may have probative value.
The examination and identification of fired and unfired cartridge cases/shotshell cases are based on the following principles:
- The various parts of the operating mechanism of firearms (firing pin, breech face, chamber, extractor, ejector, and other areas) can bear unique microscopic characteristics as a result of manufacturing processes, use, and abuse.
- These characteristics may mark the surfaces of fired and unfired cartridge cases/shotshell cases with striated marks or impressions as they are fired in or cycled through the firearm.
- These individual characteristics may be reproducible and may be uniquely identifiable with a particular firearm.
These principles and the use of comparison microscopy combine to form the conceptual and physical foundation for firearm identification.
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