Caliber (artillery)

In artillery, caliber or calibre is the internal diameter of a gun barrel, or by extension a relative measure of the length.

Rifled barrels introduce ambiguity to measurement of caliber. A rifled bore consists of alternating grooves and lands. The distance across the bore from groove to groove is greater than the distance from land to land. Projectiles fired from rifled barrels must be of the full groove to groove diameter to be effectively rotated by the rifling, but the caliber has sometimes been specified as the land to land diameter before rifling grooves were cut. The depth of rifling grooves (and the consequent ambiguity) increases in larger calibers. Steel artillery projectiles may have a forward bourrelet section machined to a diameter slightly smaller than the original land to land dimension of the barrel and a copper driving band somewhat larger than the groove to groove diameter to effectively seal the bore as it becomes enlarged by erosion during prolonged firing. United States Navy guns typically used rifling depth between one-half and one percent of caliber. Projectile bourrelet diameter specification was 0.015 inches (0.38 mm) less than land to land diameter with a minus manufacturing tolerance so average clearance was about 0.012 inches (0.30 mm). Driving band diameter was groove to groove diameter plus 0.02 inches (0.51 mm).

The length of the barrel (especially for larger guns) is often quoted in calibers. For example, US Naval Rifles 3 in (76 mm) or larger. The effective length of the barrel (from breech to muzzle) is divided by the barrel diameter to give a dimensionless quantity. As an example, the main guns of the Iowa-class battleships can be referred to as 16"/50 caliber. They are 16 inches in diameter and the barrel is 800 inches long (16 × 50 = 800). This is also sometimes indicated using the prefix /L; so for example, the most common gun for the Panzer V tank is described as a "75 mm /L70," meaning a barrel with an internal bore of 75 mm, and 5,250 mm long (17 ft 2.69 in).

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