Caliber

or Calibre . From the Latin qua librâ , “what pound,” applied first to the weight of a bullet, then to the diameter, which determined the diameter of the gun, now signifies the diameter of the bore of a cannon or any fire-arm, and is expre ed in inches or fractional parts of an inch, as a 15-inch gun; a Springfield rifle, caliber.45. Cannon are sometimes also designated by the weight of metal which they throw, as a 24-pounder.