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back sharply against the breech face or recoil plate, and this action seats the primer again….
When the material of the primer is very soft, or the breech pressure is very high, or more particularly if the soft primer has a very strong mixture in it and the vent hole is small, the metal forming the surface of the primer cup often is forced back more or less into the firing pin hole in the breech block, thus leaving a raised rim all around the firing pin impression.
The firing pin is often not fully retracted, and so it may impact the casing multiple times (Krivosta, 2006:42). Likewise, the firing pin may scrape or drag somewhat against the edge of the surface.
Also emitted from the barrel as a result of firing is gunshot residue, a mixture of partially burned and unburned particles of propellant, leftover primer mixture, and particles of metal and lubricant from the release of the bullet and its passage through the barrel. Some residue may also remain in the barrel and possibly on other internal surfaces of the gun; with time, and in the absence of cleaning, these residues can build up and alter the surface to which the bullet and cartridge case are exposed during firing.
2–C BASIC TOOLMARKS ON BALLISTICS EVIDENCE
2–C.1 Cartridge Case Markings
Breech Face Marks
Gas pressure created during the firing process exerts pressure in all directions, including forcing the head of the cartridge against the breech face. Hence, the surface area of the cartridge head may pick up negative impressions of any linear striations or other features left on the breech face when it is filed and machined. Some of these marks may register on the relatively hard cartridge brass that forms the outer ring (head stamp area) of the cartridge case, but most of the features show up in the softer surface of the primer cap. Hence, what is known as the breech face mark is the pattern of linear striations and other textural features on the surface of the primer, surrounding the indentation of the firing pin impression. Figure 2-2 illustrates the breech face marks and firing pin impression for two different firearms, one Glock and one Smith & Wesson.
Hatcher (1935:265–266) provided an early description of the breech face mark and recognized the mark’s importance as a potentially identifiable feature:
In both [semi]automatic pistols and revolvers there are certain fine tool marks or scratches left on the breech face or the metal against which the