Caliber - Cartridge Naming Conventions

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 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 calibers for small arms are usually expressed with an "x" between the width and the length; for example, 7.62x51 NATO. This indicates that the barrel diameter is 7.62 mm land to land, loaded in a case 51 mm long. Similarly, the 6.5x55 Swedish cartridge is fired from a 6.5 mm diameter barrel 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 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 lands and uses a .308" diameter (.308 times 25.4 = 7.82 mm) bullet. An exception to this rule are the proprietary cartridges used by U.S. maker Lazzeroni, 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 0.17 inches (4.5 mm) up to 0.50 inches (12.7 mm). Arms used to hunt large dangerous game, such as those used in express rifles, may be as large as 0.80 inches in 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 0.75 inches (19 mm). Paintball guns (or "markers") are typically .68 caliber (17 mm).