Rifling - Construction and Operation

Construction and Operation

A barrel of circular 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, though 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 (the diameter across the lands or high points in the rifling), or by groove diameter (the diameter across the grooves or low points in the rifling). Differences in naming conventions for cartridges can cause confusion; 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, while the ".308" refers to the groove diameter in inches (7.70 mm and 7.82 mm, respectively).

Despite differences in form, the common goal of rifling is to deliver the projectile accurately to the target. In addition to imparting the spin to the bullet, the barrel must hold the projectile securely and concentrically as it travels down the barrel. This requires that the rifling meet a number of tasks:

• It must be sized so that the projectile will swage or obturate upon firing to fill the bore.
• The diameter should be consistent, and must not increase towards the muzzle.
• The rifling should be consistent down the length of the bore, without changes in cross-section, such as variations in groove width or spacing.
• It should be smooth, with no scratches lying perpendicular to the bore, so it does not abrade material from the projectile.
• The chamber and crown must smoothly transition the projectile into and out of the rifling.

When the projectile is swaged into the rifling, it takes on a mirror image of the rifling, as the lands push into the projectile in a process called engraving. Engraving takes on not only the major features of the bore, such as the lands and grooves, but also minor features, like scratches and tool marks. The relationship between the bore characteristics and the engraving on the projectile are often used in forensic ballistics.