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 that a bullet 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 inch 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 inch projectile; or a ".22 rimfire", referring to any rimfire cartridge using a .22 caliber 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 caliber (2.5, 3.0, & 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. 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 inches (12.7 cm) and a barrel length of 50 times 5 inches = 250 inches (6.35 m). In other words, it is used to describe the approximate internal diameter of the barrel or of the projectile that is fired out of it.
Cartridge naming conventions 
Makers of early cartridge arms had to invent methods of naming the cartridges, since there was at the time no established convention. 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 inch; the bore diameter varied considerably, from .52 to .54 inch. 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 caliber 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 "x" between the width and the length; for example, 7.62x51 NATO. This indicates that the barrel diameter is 7.62 mm land to land, loaded in a case 51 mm long. Similarly, the 6.5x55 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 inch bullet (7.70 mm vs. 7.90 mm), while the .308 Winchester, dimensionally similar to (but should not be considered interchangeable with) the 7.62x51 mm NATO cartridge, is measured across the lands and uses a .308" diameter (.308 times 25.4 = 7.82 mm) bullet. An exception to this rule are the proprietary cartridges used by U.S. maker Lazzeroni, which are named based on the groove diameter in millimeters, such as the 7.82 Warbird.
Modern small arms range in bore size from approximately 0.17 inches (4.5 mm) up to 0.50 inches (12.7 mm). Arms used to hunt large dangerous game, such as those used in express rifles, may be as large as 0.80 inches in caliber. In the middle of the 19th century, muskets and muzzle-loading rifles were .58 caliber or larger; the Brown Bess flintlock, for example, had a bore diameter of about 0.75 inches (19 mm). Paintball guns (or "markers") are typically .68 caliber (17 mm).
Metric versus Imperial 
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, there is a difference of as much as 0.045 inches (1.15 mm) between the smallest and largest of the several cartridges designated as '.38 caliber'. And it may be noted that .38 inches is more than 9 1/2 mm.
|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||0.223-0.224 in||.22 Long Rifle, .222 Remington, .223 Remington, 5.56×45mm NATO, 5.7 x 28 mm, .22-250 Remington, .22 Airgun|
|.24||6 mm||0.243 in||.243 Winchester, 6 mm Remington, 6mm plastic airsoft BBs|
|.25||6.35 mm||0.25 in, 6.35 mm||.25 ACP, 6.35x16mmSR||a.k.a. .25 Auto and 6.35 mm Browning|
|.26||6.5 mm||0.264 in, 6.7 mm||6.5 x 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 x 57 mm, 7mm-08 Remington, 7 x 33 mm Sako||commonly called 7 mm|
|.30||7.62 mm||0.308 in||300 AAC Blackout, 30-06, .308 Winchester (7.62x51mm NATO), .300 Winchester Magnum, 7.82 Lazzeroni Patriot||American ".30 caliber"|
|.303||7.62 mm||0.311 in - 0.312 in||.303 British, 7.62x39, 7.62x54R, 7.62x25||Other ".30 caliber", note the mis-labelling of the Russian "7.62" rounds where neither the bore nor the groove diameter is 7.62mm|
|.323||8 mm||0.323 in||8x57mm IS, .325 WSM, 8 mm Remington Magnum, 8mm plastic (airsoft) BBs||.32 caliber rifle cartridges|
|.357||9 mm||0.355-0.357 in||.38 Special, .380 ACP, .357 Magnum, .357 SIG, .35 Remington, 9×19mm Parabellum, 9x18mm Makarov, .357 in certain new Crosman Pre-charged 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, 12.7 x 108 mm||M2 Browning machine gun and other heavy machine guns, long range rifles typified by Barrett Firearms Manufacturing products|
Shotguns are classed according to gauge, a related expression. The gauge of a shotgun refers to how many lead spheres the diameter of the bore would equal a pound. In the case of a 12-gauge shotgun, it would take twelve 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 twelve caliber." The sixteenth 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.
Caliber as measurement of length 
The length of artillery barrels has often been described in terms of multiples of the bore diameter e.g. a 4-inch gun of 50 calibers would have a barrel 50 x 4 inches = 200 inches long.
Pounds as a measure of cannon bore 
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.
However, in practice there was significant variation 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 that 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.
See also 
- List of cartridges by caliber
- List of handgun cartridges
- List of rifle cartridges
- List of the largest cannons by caliber
- Frank C. Barnes, ed. Stan Skinner. Cartridges of the World, 10th Ed. Krause Publications. ISBN 0-87349-605-1.
- Barnes, Frank C. (1997) . In McPherson, M.L. Cartridges of the World (8th Edition ed.). DBI Books. pp. 8–12. ISBN 0-87349-178-5.
- Lazzeroni Arms. "Reloading Data".
- Accurate (2000). Accurate Smokeless Powders Loading Guide (Number Two (Revised) ed.). Prescott, AZ: Wolfe Publishing. p. 392. barcode 94794 00200.
- "Pistol and Rifle Lead Bullets".
- "Rifle Bullets".
- "LeadSafe Total Copper Jacket ("TCJ") Bullet List".