Calibers

In guns, particularly firearms, caliber or calibre is the approximate internal (bore) diameter of the gun barrel, or the diameter of the projectile it shoots, in hundredths or sometimes thousandths of an inch. For example, a ".45 caliber" firearm has a barrel diameter of 0.45 inches (11 mm). 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 (5.6 mm) 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 in the world. Good performance requires a bullet to closely match the groove diameter of a barrel to ensure a good gas seal and hence maximal bullet propulsion.

While modern firearms are generally referred to by the name of the cartridge the gun is chambered for, they are still categorized 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 0.30 inches (7.6 mm) projectile; or a "22 rimfire", referring to any rimfire firearms firing cartridges with a .22 caliber projectile.

Firearm calibers outside the range of .17–.50 inches (4.3–12.7 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, and .700 (14.7, 14.9, 15.2, 17.8 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 70 caliber used in a rifle.

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