Caliber

While modern firearms are generally referred to by the name of the cartridge the gun is chambered for, they are still categorized together based on bore diameter. For example, a firearm might be described as a "30 caliber rifle", which could accommodate any of a wide range of cartridges using a roughly projectile; or as a "22 rimfire", referring to any rimfire firearms firing cartridges with a 22 caliber projectile. However, there can be significant differences in nominal bullet and bore dimensions, and all cartridges so "categorized" are not automatically identical in actual caliber. For example, 303 British firearms and projectiles are often "categorized" as ".30-caliber" alongside several dozen U.S. "30-caliber" cartridges despite using bullets of diameter while all U.S. "30-caliber" centerfire rifle cartridges use a common, standard bullet outside diameter. Using bullets larger than design specifications causes excessive pressures, while undersize bullets cause low pressures, insufficient muzzle velocities and fouling that will eventually lead to excessive pressures. Makers of early cartridge arms had to invent methods of naming 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 black powder-era cartridges used naming schemes that appeared similar, but measured entirely different characteristics; 45-70, 44-40, and 32-20 were designated by bullet diameter to hundredths of an inch and standard black powder charge in grains. Optionally, the bullet weight in grains was designated, such as 45-70-405. The .308 Winchester is measured across the grooves and uses a .308-in diameter (7.82-mm) bullet; the military-specification version is known as 7.62 × 51 mm NATO, so called because the bore diameter measured between the lands is 7.62 mm, and the cartridge has a case 51 mm long. ==Rifle caliber and cartridge conversions==

Converting a rifle to fire a different cartridge in the same bore diameter, often involves merely re-chambering the barrel to the new cartridge dimensions, if the rim diameter of the new cartridge matches that of the old cartridge. Converting a rifle to fire a different cartridge in a different caliber and bore as what it initially was, means that the barrel of the rifle will also need to be changed. Because many competitive precision rifle shooters often shoot thousands of rounds per year both for practice and competitions, and they more often reach the end of their barrel life, whereby the rifling is worn down to a point where a rifle loses some of its accuracy, the choice to make a caliber or cartridge change is often done at the same time as when a new rifle barrel is fitted to the rifle by a gunsmith. There are a few important factors to consider when converting a rifle to a different caliber or cartridge. The action of the rifle should be long enough to contain the new cartridge, the magazine should also be able to hold the new cartridge, the bolt face should be the correct diameter and the extractor the correct size to hold the head of the new cartridge. The most common of these caliber conversions on rifles, are usually done to change from a parent cartridge to a new cartridge based on it, like when converting a rifle to a 6.5 mm Creedmoor from a 308 Winchester on which it is based. ==Metric and US customary==

The following table lists some of the commonly used calibers where both metric and US customary units 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 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, that amounts to one pound (454 g (1.0 lb)) in weight. In the case of a 12-gauge (18.5 mm) 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 (15.6 mm) 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" (). While shotgun bores can be expressed in calibers (the .410 bore shotgun is measured as in diameter, unlike with rifles the actual bore diameter of a smoothbore shotgun varies significantly down the length of the barrel, with the use of chokes and back-boring. In the United Kingdom, "gauge" is referred to as "bore" and in the United States "bore" is referred to as "gauge", e.g. a "12-bore shotgun or 12-gauge shotgun" has a bore or gauge that can accommodate a lead sphere weighing 1/12th of a pound. ==As a measurement of length==

The term caliber is used as a measure of length of artillery barrels from muzzle to breech, expressed as a multiple of the bore diameter. For example, a 4-inch gun of 50 calibers would have a barrel 4 in × 50 = 200 in long (written as 4" L/50 or 4"/50). A 16-inch gun of 50 calibers (16" L/50) has a barrel length of 50 × 16 = . 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 shells to travel at an initial velocity of up to 1,800 mph (2,897 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 non-standard weights using the same scheme. See Carronade#Ordnance. From about the mid-17th until the mid-19th century, the 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 the 3-pounder, 4-pounder, 6-pounder, 8-pounder, 9-pounder, 12-pounder, 18-pounder, 24-pounder, and 32-pounder 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 , whilst the contemporary English (avoirdupois) pound massed of approximately . Thus, a French 32-pounder at the Battle of Trafalgar threw a shot with more mass than an English 32-pounder. 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 with guns, such as the 2-pounder, 6-pounder, and 17-pounder anti-tank weapons. However, this value no longer definitively related 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==