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are known for their left twist (Rinker, 2004:128). The second is the number of grooves that are cut into the barrel—normally at a depth of 0.004–0.006 inch—to create the rifling, and, correspondingly, the number of raised lands between those grooves. Historically, “no standard was established and makers used, normally, six, seven, or eight grooves”; this remains the usual range, although firearms have been fielded with as few as 2 and as many as 24 grooves (Rinker, 2004:130, 131).
Barrels also vary in the degree of twist in the rifling, which affects how much rotation is put on bullets as they pass through the barrel and exit. Rinker (2004:127) observes that “few people agree on what is the proper twist. Some people want an over stabilized bullet from a fast twist. They claim best accuracy at all ranges. Other shooters believe a fast twist builds pressure and heat and they want a slow twist for minimum stability, and they have claims to back their theory.”
Some firearms differ from conventional rifling with square-edged grooves, using polygonal rifling instead. “Polygonal rifling has no sharp edges,” and instead the raised lands in the barrel have a smooth, “rounded profile which can be difficult to discern when looking down the barrel. This type of rifling is almost exclusively manufactured using the hammer or swage process” (Heard, 1997:123).
Chamber, Breech Face, and Firing Pin
The rear section (away from the muzzle) of the barrel bore is known as the chamber; it is designed and sized to fit a specific caliber of cartridge (see Section 2–A.2). The part of the firearm against which a cartridge sits when it is placed in the chamber is the breech, and the whole assembly may be referred to as the breechblock or breech bolt.
The specific surface of the breech that makes contact with the base of the cartridge is the breech face; Figure 2-1 depicts the breech faces of two firearms. The exact steps used to form the breech assembly can vary by manufacturer, and the breech face may vary in terms of the amount of filing or polishing done on it and whether any paint or other materials is applied to it. Basic filing can create gross striation marks in linear arrangements; in others, a rotary milling operation may be applied to the breech face surface, creating a pattern of concentric circles (American Institute of Applied Science, 1982:77). These steps are crucial to the theory of firearms identification as it is random imperfections created in these machining and filing processes that is said to make the surface (and the negative impressions of said surface, left on fired cartridge casings) unique.
A hole drilled through the breech assembly holds the firing pin, a very hard steel rod that can be forced to protrude from the breech to strike the primer of a cartridge seated in the chamber. While most firing pins have a