In firearms, caliber or calibre is the approximate diameter of the bullet used, measured in inches or millimetres. 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. It is important to performance that a bullet should 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 weapons 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.

Cartridge naming conventions

Makers of early cartridge arms had to invent methods of naming^[1] the cartridges, since there was at the time no established convention. One of the early established cartridge arms was the Spencer repeating rifle, which saw service 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 black powder-era 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 introduced in 1895 for the Winchester 1894 rifle as the .30 WCF or .30 Winchester centerfire cartridge. Designating bullet weight in cartridge name fell out of favor in the early 20th century. Some of these cartridges remain popular today, such as the .45-70, .44-40, and .30-30 Winchester although the actual charges used in modern powder may differ in weight from the original.
With the growing number of cartridges chambered for new smokeless powders, the cartridges started to be named based on bullet diameter combined with some other identifier. The .30-03 and .30-06 were named for the dates of introduction, 1903 and 1906, respectively. The .45 ACP, or .45 Automatic Colt Pistol, described the developer and intended use. Other times, some liberties are taken with the bullet diameter to differentiate different cartridges; for example, the .221 Fireball, .222 Remington, and .223 Remington all use the same bullet diameter, but the cartridges are different lengths. Some cartridges use a relative length in the name, such as .22 Short 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 cartridge uses a 7.62 mm diameter bullet, loaded in a case 51 mm long. Similarly, the 6.5x55 Swedish cartridge has a bullet diameter of 6.5 mm and 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, while dimensionally similar to (but should not be considered interchangeable with) the 7.62x51 mm NATO cartridge, is measured across the grooves and uses a .308" diameter (7.62 mm) bullet. An exception to this rule are the proprietary cartridges used by U.S. maker Lazzaroni, which are named based on the groove diameter in millimeters, such as the 7.82 Warbird.^[1][2]
Modern small arms range in bore size from approximately .17 (4.5 mm) up to .50 caliber (12.7 mm). Arms used to hunt large dangerous game, such as those used in express rifles, may be as large as .80 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 .75 caliber (19 mm). Paintball guns (or "markers") are typically .68 caliber (17 mm).

Metric versus inch

The following table lists some commonly used calibers with their metric and inch equivalents. Some calibers appear more than once; due to variations in naming conventions, as well as whims of the creator of various cartridges, bullet diameters can vary quite widely from the diameter implied by the name. For example, the .38 caliber cartridges in particular vary quite a bit, covering a range of approximately 0.045 inches (1.15 mm) from smallest to largest bullet diameter.

Common calibers in inch and their metric equivalents^[3][4][5][6]

US caliber	Metric Equivalent	Typical Actual Bullet Dia.	Common cartridges	Notes
.17	4.4 mm	0.172 in	.17 Remington, .17 HMR
.177	4.5 mm	.177 lead, .175 BB	Airgun and BB gun .177 caliber
.20, .204	5 mm	0.204 in	.204 Ruger
.22, .218, .219 .220, .221, .222, .223, .224, .225, .226	5.5, 5.56, 5.7 mm	0.223-0.224 in	.22 Long Rifle, .223 Remington (5.56mm NATO), 5.7 x 28 mm
.228	none	0.228 in	.228 Ackley Magnum	Bullets formerly available from Barnes, in heavily constructed 70 and 90 grain weights for medium game use
.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
.257	6.5 mm	0.257 in, 6.527 mm	.257 Roberts, .25-06 Remington	typical 25 cal, not normally called 6.5
.26	6.5 mm	0.264 in, 6.7 mm	6.5 x 55 mm	cartridges commonly known as 6.5
.27	6.8 mm, 7 mm	0.277 in, 7.035 mm	.270 Winchester, 6.8 SPC	not called 7 mm
.28	7 mm	0.284 in, 7.213 mm	7 mm Remington Magnum, 7 x 57 mm	commonly called 7 mm
.30	7.62 mm	0.308 in	30-06, .308 Winchester (7.62mm NATO)	American ".30 caliber"
.30	7.62 mm	0.311 in	.303 British, 7.62x39, 7.62x54R	Other ".30 caliber"
.32, .327	7.65 mm	0.309 - 0.312 in	.32 ACP, .32 S&W, .327 Federal Magnum	.32 caliber handgun cartridges
.32, .325	8 mm	0.323 in	.325 WSM, 8 mm Remington Magnum, 8mm plastic (airsoft) BBs	.32 caliber rifle cartridges
.338	8.58 mm	0.338 in	.338 Lapua, .338 Winchester Magnum	.338 Rifle cartridge
.38, .380, .357, .35	9 mm	0.355-0.357 in	.38 Special, .380 ACP, .357 Magnum, .35 Remington	Generally .357 for revolvers and rifles, .355 in autoloaders
.38	10 mm	0.400 in	.38-40	Old black powder cartridge
.40	10 mm	0.400 in	.40 S&W, 10 mm Auto
.404	10.25 mm	0.423 in	.404 Jeffery
.405	10.75 mm	0.411 in	.405 Winchester
.408	10.4 mm	0.408 in	.408 Chey Tac	CheyTac Intervention
.41	10.25 mm	0.410 in	.41 Magnum .41 Action Express
.416	10.6 mm	0.416 in	.416 Barrett, .416 Remington Magnum, .416 Rigby, .416 Weatherby Magnum	Long-range sniper rounds
.43	11 mm	0.43 in Sl	.43 SL large
.44	10.8 mm	0.427 - 0.430 in	.44 Magnum
.45	11.45 mm	0.451-0.452 in	.45 ACP	Handgun .45 calibers, .451 autos and .452 in revolvers
.45	11.6 mm	0.458 in	.45-70 Government	Most rifle .45 calibers
.454	11.53 mm	0.454 in	.454 Casull	Once considered a wildcat cartridge, becoming more common
.458, .46	11.6 mm	0.458 in	.460 Weatherby, .458 Winchester Magnum
.475, .480	12 mm	0.475 in	.480 Ruger, .475 Linebaugh
.50	12.7 mm	0.50 in	.50 AE, .500 S&W, .50 Beowulf	Desert Eagle, S&W X-Frame, Alexander Arms .50 Beowulf
.50	12.95 mm	0.510 in	.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
.68	17.5 mm	0.683-0.696 in	.689 Caliber Paintball markers	Typically .689 Caliber, not called 17.5mm (Not actually a firearm)

Calibers outside the range of .17 to .50 (4.5 to 12.7 mm) do 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.^[7]

Shotguns

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.