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The term caliber (or calibre) designates the inside diameter of a tube, the diameter (outside) of a solid wire or rod, or a measurement of the length of a gun relative to its diameter.

The term most often appears with respect to firearms, as a measure of the inside diameter of the barrel in inches (or hundredths of an inch) or in millimetres. (In reference to artillery, caliber is the ratio of the barrel length to the bore diameter.)


In firearms, the caliber is the approximate diameter of bullet used. 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. This is very important when handloading, as the bullet should closely match the groove diameter of the 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."

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 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 grain. Optionally the bullet weight in grain 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 calibres 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.

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).

Caliber as measurement of length

Relationship of caliber in bore and length of gun.
The length of the barrel (especially for larger guns) is often quoted in calibers. The effective length of the barrel (from breech to muzzle) is divided by the barrel diameter to give a value. As an example, the main guns of the s can be referred to as 16"/50 caliber. They are 16 inches in diameter and the barrel is 800 inches long (16 × 50 = 800). This is also sometimes indicated using the prefix L/; so for example, the most common gun for the Panzer V tank is described as a "75 mm L/70," meaning a barrel 75 mm in diameter, and 5,250 mm long.

The bore to barrel length ratio is called caliber in naval gunnery, but is called length in army artillery. Before World War II, the US Navy used 5"/51 caliber (5"/L51) as surface-to-surface guns and 5"/25 caliber (5"/L25) as surface to air guns. By the end of World War II, the dual purpose 38_caliber_gun" href="/5"/38_caliber_gun">5"/38 caliber was standard naval armament against surface and air targets. All three had a bore diameter of 5 inches (not 5.51 or 5.25 or 5.38 as often misread).

The practical effect of long barrels for modern guns is that the projectile spends more time in the barrel before it exits, and hence more time is available for expanding gasses from the propellant charge to smoothly accelerate the projectile, bringing about a higher velocity without placing undue strain on the gun. In internal ballistics terms, if we consider the base of a projectile to be a piston propelled by the expanding gas, then by increasing barrel length we increase the area swept by the piston, and hence increase the amount of energy that can be extracted from the gas's burning. A longer barrel allows more propellant to be used, and ideally all the propellant should be combusted just before the projectile exits, to achieve maximum muzzle velocity.

Early gun barrels were short and thick, typically no more than 26 calibres, as the gunpowder propellant they used burned very quickly and violently, and hence its acceleration time was short. The new, 20th-century, slow-burning propellants such as cordite and nitrocellulose allowed a gentler prolonged acceleration, hence gun barrels were made progressively longer and thinner. Muzzle velocity was then only limited by the length of barrel the construction methods of the day allowed.

Advanced technology is necessary to design and build long gun barrels which are strong enough to withstand the forces involved in accelerating the shell to a high velocity, while remaining light enough to be reasonably mobile, rigid enough to maintain accuracy, and having a bore able to withstand many firings before needing refurbishment. In World War I 45-calibre naval gun barrels were typical, in World War II 50- to 55-calibre barrels were common, with Germany already manufacturing tank guns of 70 calibres by 1943. Today 60- to 70-calibre barrels are not uncommon, but the latest technology has allowed shorter barrels of 55 calibres to attain muzzle velocities of 5,700 feet/second, as with the Rheinmetall 120 mm tank gun.

However, such relatively low calibre ratings for modern high-velocity guns can be misleading, as many such guns fire projectiles which are much smaller than the gun bore, and relatively light, using discarding sabots while in the barrel, and hence if we were to divide the gun bore length by the actual projectile diameter we would have a number of 60 - 70 calibres. Similarly, modern high-explosive filling is far more powerful than that used in the early 20th century, resulting in lighter shells being fired for a set bore diameter compared to 100 years ago, giving higher muzzle velocity for a given barrel calibre length without sacrificing firepower.

Alternative measurements of bore diameter


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, absent an internationally accepted standard for a pound, each state's artillery park exhibited significant variation in the actual mass of the projectile for a given nominal shot weight. Thus, the country of manufacture is a significant consideration when determining bore diameters. For example, the French livre, until 1812, had a mass of 489.5g whilst the contemporary English (Avoirdupois) pound massed approximately 454g. Thus, a French 32 pdr. at Trafalgarmarker threw a shot massing 1.138 kg more 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.

English gun classes c. 1800 are given as representative examples:
gun class (pdr.) shot diameter (cm) shot volume (cm3) approx. service bore (cm) mass of projectile (kg)
2 6.04 172.76 6.64 0.90846
3 6.91 172.76 7.60 1.36028
4 7.60 230.30 8.37 1.81339
6 8.71 345.39 9.58 2.71957
9 10.00 518.28 11.00 4.08091
12 10.97 691.22 12.07 5.44269
18 12.56 1036.96 13.81 8.16499
24 13.82 1382.65 15.20 10.88696
32 15.21 1843.50 16.73 14.51572
64 19.17 3686.90 21.08 29.03063

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. While guns continued to be classed by the weight of their projectile into the middle of the 20th. century, particularly in English and latterly British service, this value no longer had any relation to the bore diameter as the projectiles themselves were no longer simple spheres and in any case were now more often hollow shells with explosive fillings rather than solid iron shot.


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. Counter-intuitively, 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 "calibre number": e.g., "shotgun of the twelve caliber." The sixteenth caliber is known as "lordly" ( ). While shotgun bores can be expressed in calibers (the .410 bore shotgun is in fact a caliber measure of .41 calibre [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.

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
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
.41 10.25 mm 0.410 in .41 Magnum .41 Action Express
.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.

Calibre in Horology

The term caliber is used to distinguish and identify precisely its provenience: the manufacturer, the size and the type of movement used within a timepiece. (Please see a listing of most Swiss movement calibres [26485] )

Aviation bombs

Some countries (the former USSR and Russian Federation, for instance) use the "caliber" term to classify aviation bombs. The Russian/Soviet bomb caliber is expressed in mass/weight units, but may not be equal to the mass/weight of the munition.

Other uses

  • In architecture, the caliber of a column is its diameter.
  • In electricity, the caliber of an instrument of measure is the maximum value it can measure.
  • In nautical parlance, the caliber of a chain is the diameter of the metal rod used to make each chain link.
  • In agriculture, produce is also often ranked by caliber (diameter); for instance, olives, peas, or eggs.
  • In typography, the caliber of a font designates the size of the eye of a character, neglecting any risers or descenders.
  • In medicine, the caliber of a tube in the body (for example, the colon) is its diameter.
  • Colloquially, the term "high caliber" is used to refer to people or employees of great competence or ability.

See also


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