Caliber, the diameter of the bore (inside) of the barrel of a firearm. The term is also used to refer to the diameter of a bullet or shell. (The exact caliber of a bullet for a given firearm is generally slightly larger than the caliber of the firearm itself. This is because the depth of the spiral grooves that make up the bore's rifling is not taken into account in expressing the bore's diameter.) The caliber of a rifle, pistol, or machine gun is given either in hundredths (or thousandths) of an inch or in millimeters. For example, the caliber of a rifle whose bore measures 30 hundredths of an inch (about 1/3 inch) can be expressed as .30 caliber or as 7.62 mm.
Shotgun sizes are expressed in gauge, based on the size of a lead ball that would fit in the muzzle. Artillery calibers are given in millimeters or inches. In large guns, especially naval guns, caliber is also a measure of length.