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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 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. When the barrel diameter is given in inches, the abbreviation "cal" is used in place of "inches". For example, a small-bore rifle with a diameter of 0.22 in is a .22 cal; however, the decimal point is generally dropped when spoken, making it "twenty-two caliber" or a "two-two caliber". Calibers of firearms can be referred to in millimeters, as in a "caliber of eighty-eight millimeters" (88 mm) or "a hundred and five-millimeter caliber gun" (often abbreviated as "105 mm gun").
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.
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).
Makers of early cartridge arms had to invent methods of naming the cartridges, since no established convention existed then. One of the early established cartridge arms was the Spencer repeating rifle, which Union forces used in the American Civil War. It was named based on the chamber dimensions, rather than the bore diameter, with the earliest cartridge called the "No. 56 cartridge", indicating a chamber diameter of .56 in; the bore diameter varied considerably, from .52 to .54 in. Later various derivatives were created using the same basic cartridge, but with smaller-diameter bullets; these were named by the cartridge diameter at the base and mouth. The original No. 56 became the .56-56, and the smaller versions, .56-52, .56-50, and .56-46. The .56-52, the most common of the new calibers, used a .50-cal bullet.
Other early white powder-era (Ballistite and Poudre blanche) cartridges used naming schemes that appeared similar, but measured entirely different characteristics; .45-70, .38-40, and .32-20 were designated by bullet diameter in hundredths of an inch and standard black powder charge in grains. Optionally, the bullet weight in grains was designated, e.g. .45-70-405. This scheme was far more popular and was carried over after the advent of early smokeless powder cartridges such as the .30-30 Winchester and .22 Long; or a relative power, such as .44 Special and .44 Magnum. Variations on these methods persist today, with new cartridges such as the .204 Ruger and .17 HMR (Hornady Magnum Rimfire).
Metric calibers for small arms are usually expressed with an "×" between the width and the length; for example, 7.62×51 NATO. This indicates the barrel diameter is 7.62 mm land to land, loaded in a case 51 mm long. Similarly, the 6.5×55 Swedish cartridge is fired from a 6.5-mm-diameter barrel and has a case length of 55 mm. The means of measuring a rifled bore varies, and may refer to the diameter of the lands or the grooves of the rifling; this is why the .303 British, measured across the lands, actually uses a .311-in bullet (7.70 mm vs. 7.90 mm), while the .308 Winchester, dimensionally similar to (but should not be considered interchangeable with) the 7.62×51 mm NATO cartridge, is measured across the grooves and uses a .308-in diameter (7.82-mm) bullet. An exception to this rule is the proprietary cartridge used by U.S. maker Lazzeroni, which is named based on the groove diameter in millimeters, such as the 7.82 Warbird.
In the middle of the 19th century, muskets and muzzle-loading rifles were .58 cal or larger; the Brown Bess flintlock, for example, had a bore diameter of about 0.75 in (19 mm). Paintball guns (or "markers") are typically .68 cal (17 mm).
The following table lists some commonly used calibers where both metric and imperial are used as equivalents. Due to variations in naming conventions, and the whims of the cartridge manufacturers, bullet diameters can vary widely from the diameter implied by the name. For example, a difference of as much as 0.045 in (1.15 mm) occurs between the smallest and largest of the several cartridges designated as ".38 caliber".
|Inch caliber||Metric caliber||Typical bullet diameter||Common cartridges||Notes|
|.20, .204||5 mm||0.204 in||.204 Ruger, 5 mm Remington Rimfire Magnum|
|.22||5.56 mm||0.220–0.224 in (5.6–5.7 mm)||.22 Long Rifle, .222 Remington, .223 Remington, 5.56×45 mm NATO, 5.45×39 mm, 5.7×28 mm||5.45×39 mm bullet is actually 5.6 mm (AK-74)|
|.24||6 mm||0.243 in||.243 Winchester, 6 mm Remington, 6 mm plastic airsoft BBs|
|.25||6.35 mm||0.25 in, 6.35 mm||.25 ACP, 6.35×16 mmSR||or .25 auto and 6.35 mm Browning|
|.26||6.5 mm||0.264 in, 6.7 mm||6.5×55 mm, .260 Remington||cartridges commonly known as '6.5 mm'|
|.27||6.8 mm||0.277 in, 7.035 mm||.270 Winchester, 6.8 SPC|
|.28||7 mm||0.284 in, 7.213 mm||.280 Remington, 7 mm Remington Magnum, 7×57 mm, 7mm-08 Remington||commonly called '7 mm'|
|.30, .308||7.8 mm||0.308 in||300 AAC Blackout, .30-06, .300 Winchester Magnum, 7.82 Lazzeroni Patriot, .30-30 Winchester, .308 Winchester, 7.62×51mm NATO||American ".30 caliber"|
|.303, .31||7.9 mm||0.31–0.312 in (7.9–7.9 mm)||.303 British, 7.62×39, 7.62×54R, 7.62×25, 7.7x58||7.62×54R is actually 7.92 mm (Mosin, SVD, PKM, etc.) The same applies to 7.62×39 (AK-47, AKM, etc.)|
|.323||8 mm||0.323 in||8×57mm IS, .325 WSM, 8 mm Remington Magnum, 8 mm plastic (airsoft) BBs||.32 caliber rifle cartridges|
|.338||8.6 mm||0.338 in||.338 Lapua||C14 Timberwolf (Canadian Forces)|
|.357||9 mm||0.355–0.357 in (9.0–9.1 mm)||.38 Special, .380 ACP, .357 Magnum, .357 SIG, .35 Remington, 9×19 mm Parabellum, 9×18 mm Makarov, .357 in certain new Crosman precharged pneumatic (PCP) airguns.||Handgun cartridges known as "38" are .357 caliber. Generally .357 for revolvers and rifles, .355 in autoloaders|
|.40||10 mm||0.400 in||.40 S&W, 10 mm auto|
|.45||11.43 mm||0.450 in||.45 ACP, .45 GAP, .454 Casull, .45 Long Colt|
|.50||12.7 mm||0.510 in (12.95 mm)||.50 BMG, .50 Action Express, 12.7×108 mm||M2 Browning machine gun and other heavy machine guns, long-range rifles typified by Barrett products|
Shotguns are classed according to gauge, a related expression. The gauge of a shotgun refers to how many lead spheres, each with a diameter equal to that of the bore, amount to one pound in weight. In the case of a 12-gauge shotgun, it would take 12 spheres the size of the shotgun's bore to equal a pound. A numerically larger gauge indicates a smaller barrel: a 20-gauge shotgun requires more spheres to equal a pound; therefore, its barrel is smaller than the 12-gauge. This metric is used in Russia as "caliber number": e.g., "shotgun of the 12 caliber." The 16th caliber is known as "lordly" (Russian: барский). While shotgun bores can be expressed in calibers (the .410 bore shotgun is in fact a caliber measure of .41 caliber [11 mm]), the nature of shotshells is such that the barrel diameter often varies significantly down the length of the shotgun barrel, with various levels of choke and backboring.
The length of artillery barrels has often been described in terms of multiples of the bore diameter e.g. a 4-in gun of 50 calibers would have a barrel 50 x 4 in = 200 in long. A 50 caliber 16 inch navy gun (16 inch diameter shell), has a barrel length (muzzle to breech) of 16x50=800 in (66 ft 8 in). Both 14-in and 16-in navy guns were common in World War II. The British Navy insisted on 50-cal guns on navy ships as it would allow 1,900- to 2,700-lb (861- to 1225-kg) shells to travel at 1800 mi/h (2896 km/h) to a distance of 26 mi (42 km).
Smoothbore cannon and carronade bores are designated by the weight in imperial pounds of round solid iron shot of diameter to fit the bore. Standard sizes are 6, 12, 18, 24, 32, and 42 pounds, with some 68-pound weapons, and other nonstandard weapons using the same scheme. See Carronade#Ordnance.
From about the middle of the 17th century until the middle of the 19th century, measurement of the bore of large gunpowder weapons was usually expressed as the weight of its iron shot in pounds. Iron shot was used as the standard reference because iron was the most common material used for artillery ammunition during that period, and solid spherical shot the most common form encountered. Artillery was classified thereby into standard categories, with 3 pounders (pdr.), 4 pdr., 6 pdr., 8 pdr., 9 pdr., 12 pdr., 18 pdr., 24 pdr., and 32 pdr. being the most common sizes encountered, although larger, smaller and intermediate sizes existed.
In practice, though, significant variation occurred in the actual mass of the projectile for a given nominal shot weight. The country of manufacture is a significant consideration when determining bore diameters. For example, the French livre, until 1812, had a mass of 489.5 g whilst the contemporary English (avoirdupois) pound massed approximately 454 g. Thus, a French 32 pdr. at the Battle of Trafalgar threw a shot with 1.138 kg more mass than an English 32 pdr.
Complicating matters further, muzzle-loaded weapons require a significant gap between the sides of the tube bore and the surface of the shot. This is necessary so the projectile may be inserted from the mouth to the base of the tube and seated securely adjacent the propellant charge with relative ease. The gap, called windage, increases the size of the bore with respect to the diameter of the shot somewhere between 10% and 20% depending upon the year the tube was cast and the foundry responsible.
|gun class (pdr.)||shot diameter (cm)||shot volume (cm3)||approx. service bore (cm)||mass of projectile (kg)|
The relationship between bore diameter and projectile weight was severed following the widespread adoption of rifled weapons during the latter part of the 19th century. Guns continued to be classed by projectile weight into the mid-20th century, particularly in British service. However, this value no longer had any relation to bore diameter, since projectiles were no longer simple spheres—and in any case were more often hollow shells filled with explosives rather than solid iron shot.