Cal or Caliber
  • System of measurement for the internal bore diameter of a rifled-barreled firearm (rifle or pistol) based on the decimal part of an inch. For example, .25 calibre and .250 calibre both signify a bore size of 1/4 inch. American calibre designations refer to the distance from land to land, not groove to groove. Ammunition companies' marketing departments occasionally take liberties with exact measurements. For example, a .270 Winchester bullet actually measures .277 inch in diameter.