The marks on fired cartridge cases and shotshell cases reflect both class and individual characteristics of striated and impressed toolmarks.
In addition to class and individual marks, there is a possibility for the
appearance of a third category of mark - subclass characteristics. These
three categories are defined below:
Extractor, ejector and firing pin aperture
of a Smith and Wesson pistol
Courtesy of Jack Dillon
Click for larger image
Subclass characteristics should not be confused with individual characteristics. Subclass characteristics are similarities appearing in consecutively manufactured parts of some firearms. These subclass characteristics can reflect the method used by the manufacturer over a particular period of time.
In the case of a "no gun" submission, the influence of subclass characteristics may or may not be eliminated.
Faced with the possibility of subclass influence, the examiner should
- research the manufacturing processes related to a particular make and model,
- consult with peers regarding the firearm in question,
- research pertinent literature, especially the AFTE Journal.