FIREARM BARRELS
The interior surface of the barrels of the irearms discussed so far (handguns and rifles, but not shotguns) are rifled with a series of ridges and valleys, called lands and grooves, respectively, that spiral the length of barrel (see Figure 1). The lands dig into the bullet surface as it travels down the barrel, imparting spin to stabilize the bullet’s light once it leaves the barrel. This creates land and groove impressions on the bullet surface as well as impressions of the microscopic imperfections of the interior barrel surface called striations or striae (see Figure 2).
FIGURE 1 Spiral grooves are cut into the inner surface of a firearm barrel to impart spin to the bullet as it leaves the barrel, stabilizing its flight. The raised portions between the grooves are called lands.
FIGURE 2 Imperfections in the surface of the tool that cuts the rifling grooves are transferred to the barrel’s inner surface. These striations or striae are then transferred to a bullet’s outer surface when it is fired. Striations are considered to be unique to a particular barrel, through manufacturing and use. These are on lead bullets fired from a revolver.
During manufacture of a barrel, a hole is drilled down the length of a steel bar of the proper size for the intended irearm. The grooves are cut into the barrel by either a large segmented tool, called a broach, or a rifling button, a stiff metal rod with a flanged tip, which is run down the length of the hole. When the grooves are cut, they are cut in a spiral of a certain direction or twist (right-handed/clockwise or left-handed/counterclockwise); this is what spins the bullet and creates a stable flight path. Some manufacturers produce barrels with four grooves, some with ive or six, depending on the design and desired performance of the firearm. The interior or bore diameter of a rifled barrel is the diameter of a circle that touches the tops of the lands. The caliber of a firearm used to mean the same thing as bore diameter but now refers mostly to the size of a particular ammunition cartridge; irearms are still referred to in their nominal caliber, however. A barrel’s internal diameter is an exact measurement, while caliber is an approximation; the barrel of a 0.38 firearm may actually measure between 0.345 and 0.365 in (also note that calibers do not use the zero before the decimal). The caliber of American and British ammunition is typically measured in inches, and all others are measured in millimeters (a Smith and Wesson 0.32 vs a Beretta 9 mm). Shotguns can ire numerous projectiles, called pellets or “shot” of varying sizes (see Figure 3 and Table 1); they may also ire single projectiles called “slugs.” A single-barrel shotgun can be either single-shot (manually loaded) or repeating-shot in design (with a spring-loaded auto-feeder or manual pump feeder with a reservoir of three to ive shells). The interior of a shotgun barrel is smooth so that nothing deflects or slows down the pellets as they traverse its length. The muzzle of a shotgun barrel may be constricted by the manufacturer to produce a choke, which helps to keep the pellets grouped longer once they leave the barrel. The finluence of choke on the shot pattern increases with the distance the pellets travel; the range of a shotgun is, compared to rifles, short, but the choke can improve the chance of hitting targets at near to mid-ranges (see Table 2). The choke may also be modified by barrel inserts.
FIGURE 3 A shotgun is essentially a rifle that doesn’t shoot bullets: instead, it fires many small, round pellets or a single large slug. Therefore, shotgun barrels do not have rifling and have a limited effective firing range.
Table 1 The Size of Pellets Is Organized Numerically, Except for the Two Largest, “BB” and “00” (or “Double Ought”) Buckshot
Table 2 Choke is the Measure of Constriction of a Shotgun Barrel, Intended to Group the Pellets, and Produce a Tighter Pattern at Impact. Some Shotgun Barrels May Have their Choke Modified by a Removable Insert
Table 3 The Size of a Shotgun Barrel is Measured in Gauge, Except for the Smallest, Which is Labeled as “410” (“Four-ten”) Because the Barrel’s Internal Diameter is 0.410-in Wide
The diameter of the shotgun barrel is called gauge and is the number of lead balls with the same diameter as the barrel that would weigh 1 pound. For example, 12 lead balls, which together weigh 1 pound, have the same diameter as the interior of a 12-gauge shotgun (about 0.729 in). The exception to this system is the so called 410-gauge shotgun, which has its bore diameter measured in inches (0.410 in) (see Table 3).
