In
guns, particularly
firearms,
caliber or
calibre is the approximate internal diameter of the
barrel, or the diameter of the projectile it fires, in hundredths or sometimes thousandths of an
inch. For example, a 45 caliber firearm has a barrel diameter of .45 of an inch. Barrel diameters can also be expressed using
metric dimensions, as in "9mm pistol." When the barrel diameter is given in inches, the abbreviation "cal" (for "caliber") can be used. For example, a small-bore rifle with a diameter of 0.22 inches can be referred to as 22 cal or spoken: "twenty-two caliber" or a "two-two caliber rifle."
In a
rifled barrel, the distance is measured between opposing
lands or
grooves; groove measurements are common in cartridge designations originating in the United States, while land measurements are more common elsewhere. Good performance requires a bullet to closely match the groove diameter of a barrel to ensure a good seal.
While modern cartridges and
cartridge firearms are generally referred to by the cartridge name, they are still lumped together based on bore diameter. For example, a firearm might be described as a "30 caliber rifle", which could be any of a wide range of cartridges using a roughly .30-in projectile; or a "22 rimfire", referring to any
rimfire cartridge using a 22-cal projectile.
Firearm calibers outside the range of 17 to 50 (4.5 to 12.7 mm) exist, but are rarely encountered.
Wildcat cartridges, for example, can be found in 10, 12, and 14 cal (2.5, 3.0, and 3.6 mm), typically used for short-range
varmint hunting, where the high-velocity, lightweight bullets provide devastating
terminal ballistics with little risk of ricochet. Larger calibers, such as .577, .585, .600, .700, and .729 (14.7, 14.9, 15.2, 17.8, & 18.5 mm) are generally found in proprietary cartridges chambered in
express rifles or similar guns intended for use on dangerous game.
[1] The
.950 JDJ is the only known cartridge beyond 79 caliber used in a rifle.
In some contexts, e.g. guns aboard a warship, "caliber" is used to describe the barrel length as multiples of the bore diameter. A "5-inch 50 calibre" gun has a bore diameter of 5 in (12.7 cm) and a barrel length of 50 times 5 in = 250 in (6.35 m).
