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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" can be used. For example, a small-bore rifle with a diameter of 0.22 inches can be referred to as .22 or a .22 cal; however, the decimal point is generally dropped when spoken, making it a "twenty-two caliber" or a "two-two caliber rifle".

From left: .50 BMG, .300 Win Mag, .308 Winchester, 7.62×39mm, 5.56×45mm NATO, .22LR (Note that the 5.56x45mm NATO round and the .22LR round have the same diameter bullets but very different cartridges)
A .45 ACP hollowpoint (Federal HST) with two .22 LR cartridges for comparison
Side on view of Sellier & Bellot .45-cal ACP cartridge with a metric ruler for scale

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 or a .22 cal; however, the decimal point is generally dropped when spoken, making it a "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. The .950 JDJ is the only known cartridge beyond 79 caliber used in a rifle.

Referring to artillery, "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). The main guns of the USS Missouri (Iowa Class Battleship) are 16" 50 caliber.

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