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The term 'caliber' or 'calibre' designates the interior diameter of a tube or the exterior diameter of a wire or rod. It comes from the Italian ''calibro'', itself from ''qālib'' (قالب), Arabic word for ''mold''.
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.

Contents

Firearms

Cartridge naming conventions

Caliber as measurement of length

Alternative measurements of bore

Metric vs Inch

Rockets and missiles

Aviation bombs

Other uses

See also

References

Firearms

In firearms, the caliber is the diameter of the inside of the barrel. 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.
When the barrel diameter is given in inches, the abbreviation "cal" is used in place of "inches". For example, a (smallbore) 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 calibre"''.
Calibers of weapons can be referred to in metric in millimeters, as in a "caliber of eighty-eight millimetres" (88 mm) or "a hundred and five-millimetre caliber gun" (sometimes 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 barrel length, with the earliest cartridge called the "No. 56 cartridge", indicating a chamber diameter of .56 inches; the bore diameter varied considerably, from .52 to .54 inches. 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 a similar looking naming scheme, but measured entirely different characteristics. This scheme was far more popular, and was used into the advent of early smokeless powder cartridges. The cartridge would be described by the bullet diameter, in hundredths of an inch, and the powder charge in grains. Some of these cartridges remain popular today, such as the .45-70, .44-40, and .30-30 Winchester.
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 date of introduction, 1903 and 1906 respectively. The .45 ACP, or ''.45 Automatic Colt Pistol'', described the developer and intended use. Other times some liberty is 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 7.62mm diameter bullet, loaded in a case 51mm long. Similarly, the 6.5x55 Swedish cartridge has a bullet of 6.5mm, and a case length of 55mm. 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 identical to the 7.62x51mm NATO cartridge, is measured across the grooves, and uses a .308" diameter (7.82 mm) bullet.
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 are typically .68 caliber (17 mm).

Caliber as measurement of length

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 ''Iowa'' class battleships 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 IV tank is described as a "75 mm L/48", meaning a barrel 75 mm in diameter, and 3,600 mm long.

Alternative measurements of bore

Measurement of the bore of large weapons was often expressed in pounds. The weapon would be named according to the weight of a sphere of lead of the same diameter as the bore. The density of lead was used because it is a traditional material for projectiles.
This leads to certain guns being referred to as 6-pounder, 25-pounder and so forth. However this relationship between calibre and projectile weight changed with the introduction of the cylindrical rifled shell. The gun continued to be named by the weight of projectile it threw although this no longer gave a direct indication of the barrel size.
Shotguns are named 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. Counterintuitively, 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": "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 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''.

Metric vs Inch

Here are some commonly used calibers with their metric and inch equivalents. Some calibers appear more than once; the bullet used in a .38 caliber cartridge in particular varies quite a bit, up to 0.045 inches (1.15 mm) depending on the specific cartridge.

Common calibers in inch and their metric equivalents ^[2]

US caliber	Metric Equivalent	Typical Actual Bullet Dia.	Common cartridges	Notes
.17	4.4mm	.172	.17 Remington, .17 HMR
.17	4.5mm	.177 lead, .175 BB	Airgun and BB gun .17 caliber
.20, .204	5mm	.204	.204 Ruger
none	5.45	.218	5.45 x 39 mm	Soviet block bullet diameter
.22, .220, .221, .222, .223	5.5, 5.56, 5.7mm	.223-.224	.223 Remington (5.56 Nato), 5.7 x 28 mm
.24	6mm	.243	.243 Winchester, 6 mm Remington
.25	6.5mm	.257, 6.527mm	.257 Roberts, .25-06 Remington	typical 25 cal, not normally called 6.5
.26	6.5mm	.264, 6.7mm	6.5 x 55 mm	cartridges commonly known as ''6.5''
.27	6.8mm, 7mm	.277, 7.035mm	.270 Winchester, 6.8 SPC	not called 7mm
.28	7mm	.284, 7.213mm	7mm Remington Magnum, 7 x 57 mm	commonly called ''7mm''
.30	7.62mm	.308	.308 Winchester (7.62 Nato)	American ".30 caliber"
.30	7.62mm	.311	.303 British, 7.62 x 39 mm	Other ".30 caliber"
.32	7.65mm	.312	.32 ACP
.325	8mm	.323	.325 WSM, 8 mm Remington Magnum
.38, .357, .35	9mm	.355-.357	.38 Special, .357 Magnum, .35 Remington	Generally .357 for revolvers and rifles, .355 in autoloaders
.38	10mm	.400	.38-40	Old black powder wildcat cartridge
.40	10mm	.400	.40 S&W, 10 mm Auto
.41	10.25mm	.410	.41 Magnum .41 Action Express
.44	10.8mm	.429	.44 Magnum
.45	11.45mm	.451-.452	.45 ACP	Handgun .45 calibers, .451 autos and .452 in revolvers
.45	11.6mm	.458	.45-70 Government	Most rifle .45 calibers
.458, .46	11.6mm	.458	.460 Weatherby, .458 Winchester Magnum
.475, .480	12mm	.475	.480 Ruger, .475 Linebaugh
.50	12.7mm	.50	.50 AE, .50 BMG	Desert Eagle, M2 Browning Machine Gun

Rockets and missiles

Dimensions of rockets and missiles, particularly the location of the centre of pressure, are usually expressed in calibres, where one calibre is equal to the body diameter.

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.
Agricultural 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 horology, the term is used to distinguish the size and type of movement used within a timepiece.
Colloquially, the term "high caliber" is used to refer to people or employees of great competence or ability.

See also

★ Gauge

★ List of cartridges by caliber

★ List of handgun cartridges

★ List of rifle cartridges

References

1. Cartridges of the World, , Frank C., Barnes, DBI Books, 1997,
2. Accurate Smokeless Powders Loading Guide, , , Accurate, Wolfe Publishing, 2000, Barcode 94794 00200