Caliber
The bore diameter of a firearm or the diameter of its bullet.
Caliber describes the inside diameter of the bore (and hence the bullet diameter), measured in inches (.45, .308, .22) or millimeters (9mm, 5.56mm). Cartridges are usually named after their caliber, often with a qualifier — .308 Winchester, 9mm Luger, .223 Remington — because many cartridges share a caliber but differ in case dimensions and pressure.