Rifled

Design

A barrel of circular bore cross-section is not capable of imparting a spin to a projectile, so a rifled barrel has a non-circular cross-section. Typically, the rifled barrel contains one or more grooves that run down its length, giving it a cross-section resembling an internal gear. However, it can also take the shape of a polygon, usually with rounded corners. Since the barrel is not circular in cross-section, it cannot be accurately described with a single diameter. Rifled bores may be described by the bore diameter or land diameter (the diameter between lands or high points in the rifling), or by groove diameter (the diameter between grooves or low points in the rifling). Differences in naming conventions for cartridges can confuse; for example, the projectiles of the .303 British are actually slightly larger in diameter than the projectiles of the .308 Winchester, because the ".303" refers to the bore diameter in inches (bullet is .312), while the ".308" refers to the bullet diameter in inches (7.92 mm and 7.82 mm, respectively).