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Caliber (artillery)
In artillery, caliber or calibre〔difference in British English and American English spelling〕 is the internal diameter of a gun barrel, or by extension a relative measure of the length.
==Rifled barrels==
Rifled barrels introduce ambiguity to measurement of caliber. A rifled bore consists of alternating grooves and lands. The distance across the bore from groove to groove is greater than the distance from land to land. Projectiles fired from rifled barrels must be of the full groove to groove diameter to be effectively rotated by the rifling, but the caliber has sometimes been specified as the land to land diameter before rifling grooves were cut. The depth of rifling grooves (and the consequent ambiguity) increases in larger calibers. Steel artillery projectiles may have a forward bourrelet section machined to a diameter slightly smaller than the original land to land dimension of the barrel and a copper driving band somewhat larger than the groove to groove diameter to effectively seal the bore as it becomes enlarged by erosion during prolonged firing. United States Navy guns typically used rifling depth between one-half and one percent of caliber. Projectile bourrelet diameter specification was less than land to land diameter with a minus manufacturing tolerance so average clearance was about . Driving band diameter was groove to groove diameter plus .
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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