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the diameter of the bore of a firearm and also the diameter of a shell (bullet); one of the basic measures of the power of a firearm. Caliber is determined by measuring the inside barrel diameter for a smoothbore gun, the distance between opposite ribs for a rifled gun, and the maximum cross section for shells (bullets). Tapered-bore guns have inlet and outlet calibers.
A scale with the diameters of stone and cast-iron (shot) balls was first developed in Nuremberg in 1540. In Russia, Peter I established his own unit of measurement—the artillery pound— a cast-iron ball with a diameter of 2 inches weighing 115 zolotniks (approximately 480 grams). On the basis of this artillery pound a scale was created for smoothbore artillery, in which a 3-pound ball corresponded to a caliber of 2.8 inches (70 mm) and a 12-pound ball corresponded to a caliber of 4.7 inches (120 mm). Caliber was first designated in linear measures (inches and lines) in Russia in 1877, including a 3-inch cannon, 6-inch cannon, and 3-line rifle.
The calibers of all types of modern weapons are usually designated in millimeters. The length of a gun barrel in artillery is more often expressed in caliber—the length of the gun barrel is equal to as many calibers as the number of times the bore diameter goes into the barrel length. The caliber of smoothbore hunting rifles is expressed by the quantity of round pellets, equal in mass and diameter, which are cast from 400 grams of pure lead and go into the gun bore without play. For example, if 12, 16, 20, or more pellets are cast, then the caliber of these guns will be expressed by the gauges 12, 16, 20, and so forth, respectively.