of the sources of variability in ballistic toolmarks, but also in assessing the feasibility of implementing technologies like wide-scale ballistic imaging or microstamping.
Firearms come in a wide array of designs and specific makes, and each represents a complex assemblage of numerous constituent parts. In this section we focus on the parts most central to the basic firing assembly since the interest is in toolmark creation. Due to their widespread use in crime, we also discuss some terminology in the specific context of handguns, as in differentiating between revolvers and pistols.
Gun barrels are manufactured from solid pieces of steel whose composition is carefully selected for its chemical and metallurgical properties. A first step of the process, drilling, results in a comparatively rough hole of uniform diameter extending from one end of the barrel to the other. Next the barrel is bored with a reamer, designed to produce as smooth a surface as possible on the inside of the barrel. The interior surface or bore bears numerous scars and scratches from this drilling process; it is these random imperfections—more so than subsequent steps—that are said to account for individual characteristics on fired bullets (Heard, 1997:124–125).
Barrels are further subjected to a rifling process, creating a pattern of grooves on the inside the barrel. This rifling is essential to the firing accuracy of the weapon; as it is forced out of the barrel by gas pressure, the bullet impacts with the barrel rifling and is given a rotation—somewhat akin to the spin on a thrown football—that gives the bullet a more direct flight. Some weapons, typically shotguns, have no rifling (“smoothbore”). Most handguns and rifles have a spiral pattern of rifling to improve their accuracy. The rifling may be created by forcing a carbide button through the reamed barrel; it is the normal wear on this button, as many riflings are performed, that is said to impart individual microscopic variability in markings in the barrel (along with residual scars or imperfections from the original drilling). Additional steps in the process to finish a barrel include heat treating (to impart hardness) and cleaning.
Across manufacturers, barrels can vary in two fundamental features, each of which are basic class characteristics (see Section 3–B.1). The first is the direction in which the grooves in the barrel twist, whether left- or right-handed. Most U.S. makers use a right twist, although Colt revolvers