(1) The term ‘calibre’ is related to shooting. Generally, it refers to the internal diameter of a gun. In small arms, calibre refers to the internal diameter of a firearm's barrel or a cartridge's bullet, usually expressed in millimeters or hundredths of an inch; in measuring rifled barrels this may be measured across the lands or grooves or; a specific cartridge for which a firearm is chambered, such as .44 Magnum.