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 caliber".
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" (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.
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 7.62 mm diameter bullet, loaded in a case 51 mm long. Similarly, the 6.5x55 Swedish cartridge has a bullet 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 US maker Lazzaroni, which are named based on the groove diameter in millimeters, such as 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).
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 x 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 5250 mm long.
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" (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.
|US caliber||Metric Equivalent||Typical Actual Bullet Dia.||Common cartridges||Notes|
|.17||4.4 mm||.172||.17 Remington, .17 HMR|
|.177||4.5 mm||.177 lead, .175 BB||Airgun and BB gun .177 caliber|
|.20, .204||5 mm||.204||.204 Ruger|
|.22, .218, .219 .220, .221, .222, .223, .224, .225, .226||5.5, 5.56, 5.7 mm||.223-.224||.22 Long Rifle, .223 Remington (5.56mm NATO), 5.7 x 28 mm|
|.228||none||.228||.228 Ackley Magnum||Bullets formerly available from Barnes, in heavily constructed 70 and 90 grain weights for medium game use|
|.24||6 mm||.243||.243 Winchester, 6 mm Remington, 6mm plastic (airsoft) BBs|
|.25||6.5 mm||.257, 6.527 mm||.257 Roberts, .25-06 Remington||typical 25 cal, not normally called 6.5|
|.26||6.5 mm||.264, 6.7 mm||6.5 x 55 mm||cartridges commonly known as 6.5|
|.27||6.8 mm, 7 mm||.277, 7.035 mm||.270 Winchester, 6.8 SPC||not called 7 mm|
|.28||7 mm||.284, 7.213 mm||7 mm Remington Magnum, 7 x 57 mm||commonly called 7 mm|
|.30||7.62 mm||.308||30-06 .308 Winchester (7.62mm NATO)||American ".30 caliber"|
|.30||7.62 mm||.311||.303 British, 7.62x39||Other ".30 caliber"|
|.32, .327||7.65 mm||.309 - .312||.32 ACP, .32 S&W, .327 Federal Magnum||.32 caliber handgun cartridges|
|.32, .325||8 mm||.323||.325 WSM, 8 mm Remington Magnum||.32 caliber rifle cartridges|
|.38, .357, .35||9 mm||.355-.357||.38 Special, .357 Magnum, .35 Remington||Generally .357 for revolvers and rifles, .355 in autoloaders|
|.38||10 mm||.400||.38-40||Old black powder cartridge|
|.40||10 mm||.400||.40 S&W, 10 mm Auto|
|.404||10.25 mm||.423||.404 Jeffery|
|.405||10.75 mm||.411||.405 Winchester|
|.41||10.25 mm||.410||.41 Magnum .41 Action Express|
|.416||10.6 mm||.416||.416 Rigby|
|.44||10.8 mm||.427 - .430||.44 Magnum|
|.45||11.45 mm||.451-.452||.45 ACP||Handgun .45 calibers, .451 autos and .452 in revolvers|
|.45||11.6 mm||.458||.45-70 Government||Most rifle .45 calibers|
|.454||11.53 mm||.454||.454 Casull||Once considered a wildcat cartridge, becoming more common|
|.458, .46||11.6 mm||.458||.460 Weatherby, .458 Winchester Magnum|
|.475, .480||12 mm||.475||.480 Ruger, .475 Linebaugh|
|.50||12.7 mm||.50||.50 AE, .500 S&W, .50 Beowulf||Desert Eagle, S&W X-Frame, Alexander Arms .50 Beowulf|
|.50||12.7 mm||.510||.50 BMG, 12.7 x 108 mm||M2 Browning machine gun and other heavy machine guns, long range rifles typified by Barrett Firearms Company products|
|.68||17.5 mm||.683-.696||.689 Caliber Paintball Guns||Typically .689 Caliber, not called 17.5mm|
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.