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In guns, particularly firearms, caliber or calibre is the approximate internal diameter of the barrel, or the diameter of the projectile it fires, usually shown in millimeters, or in hundredths or thousandths of an inch. When expressed in inches in writing or print, it is shown in terms of a decimal fraction: .45 caliber, for example. When the barrel diameter is given in inches, the abbreviation "cal" can be used. For example, a small-bore rifle with a diameter of 0.22 inches can be called a .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. However, when caliber is expressed in millimeters, this is noted, as in, "9mm pistol." 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. 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 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 calibers for small arms are usually expressed with an "×" between the width and the length; for example, 7.62×51 NATO. This indicates the barrel diameter is 7.62 mm land to land, loaded in a case 51 mm long. Similarly, the 6.5×55 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-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.〔〔(【引用サイトリンク】title=Reloading Data )〕 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). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Caliber」の詳細全文を読む スポンサード リンク
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