Caliber

From left: .50 BMG, .300 Win Mag, .308 Winchester, 7.62×39mm, 5.56×45mm NATO, .22LR (Note that the 5.56x45mm NATO round and the .22LR round have the same diameter bullets but very different cartridges)

A .45 ACP hollowpoint (Federal HST) with two .22 LR cartridges for comparison

Side on view of Sellier & Bellot .45-cal ACP cartridge with a metric ruler for scale

In guns, particularly firearms, caliber or calibre is the approximate internal diameter of the barrel, or the diameter of the projectile it fires, in hundredths or sometimes thousandths of an inch. For example, a 45 caliber firearm has a barrel diameter of .45 of an inch. Barrel diameters can also be expressed using metric dimensions, as in "9mm pistol." When the barrel diameter is given in inches, the abbreviation "cal" (for "caliber") can be used. For example, a small-bore rifle with a diameter of 0.22 inches can be referred to as .22 or a .22 cal; however, the decimal point is generally dropped when spoken, making it a "twenty-two caliber" or a "two-two caliber rifle".

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 a bullet to closely match the groove diameter of a barrel to ensure a good seal.

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-in projectile; or a "22 rimfire", referring to any rimfire cartridge using a 22-cal 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 cal (2.5, 3.0, and 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.^[1] The .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 in (12.7 cm) and a barrel length of 50 times 5 in = 250 in (6.35 m).

Cartridge naming conventions

Makers of early cartridge arms had to invent methods of naming^[2] the cartridges, since no established convention existed then. 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 in; the bore diameter varied considerably, from .52 to .54 in. 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-cal 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 diameters for small arms refer to cartridge dimensions and are usually expressed with an "×" between the width and the length; for example, 7.62×51 NATO. This indicates the bullet diameter is 7.62mm, loaded in a case 51mm long. Similarly, the 6.5×55 Swedish cartridge has a bullet width of 6.5 mm 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-in bullet (7.70 mm vs. 7.90 mm), while the .308 Winchester, dimensionally similar to (but should not be considered interchangeable with) the 7.62×51mm NATO cartridge, is measured across the grooves and uses a .308-in diameter (7.82-mm) bullet. An exception to this rule is the proprietary cartridge used by U.S. maker Lazzeroni, which is named based on the groove diameter in millimeters, such as the 7.82 Warbird.^[2][3]

In the middle of the 19th century, the majority of muskets and muzzle-loading rifles were 50 cal or larger; the Brown Bess flintlock, for example, had a bore diameter of 0.75 in (19 mm). Paintball guns (or "markers") are typically .68 cal (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, a difference of as much as 0.045 in (1.15 mm) occurs between the smallest and largest of the several cartridges designated as ".38 caliber".

Shotguns

Shotguns are classed according to gauge, a related expression. The gauge of a shotgun refers to how many lead spheres, each with a diameter equal to that of the bore, amount to one pound in weight. In the case of a 12-gauge shotgun, it would take 12 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 12 caliber." The 16th 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 4 in x 50 = 200 in long. A 50 caliber 16 inch gun (16 inch diameter shell), has a barrel length (muzzle to breech) of 50 x 16 = 800 in (66 ft 8 in). Both 14-in and 16-in navy guns were common in World War II. The British Royal Navy insisted on 50-cal guns on ships as it would allow 1,900 to 2,700 lb (860 to 1,220 kg) shells to travel at 1,800 mph (2,896 km/h) to a distance of 26 mi (42 km).

Pounds as a measure of cannon bore

Smoothbore cannon and carronade bores are designated by the weight in imperial pounds of spherical 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.

In practice, though, significant variation occurred 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 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.

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

References

External links

• Search for guns by their caliber