cartridge presses when it is being fired. These marks are quite pronounced on metal surfaces that have been finished by a file as is commonly done on the breech face of the average [semi]automatic pistol or revolver. Examined under a microscope this surface appears to consist of a number of ridges or scratches, and when the cartridge is fired, the primer, being of copper or brass, which is much softer than the steel of the breech face, will take the impression of these fine ridges.
In gross appearance, features in the breech face impression may fall into some general categories depending on the specific filing or polishing steps used by the manufacturer. Straight filing creates linear features; other breech face impressions may feature cross-hatching or circular patterns. For example, Kennington (1995) documents the class of 9mm pistols for which the rotary cutting tool used in milling the breech face not only leaves distinctive arched markings that are impressed on the primer surface, but