Caliber

n.
  1. (Gunnery) The diameter of the bore, as a cannon or other firearm, or of any tube; or the weight or size of the projectile which a firearm will carry; as, an 8 inch gun, a 12-pounder, a 44 caliber.
  2. The diameter of round or cylindrical body, as of a bullet or column.
  3. Fig.: Capacity or compass of mind.
    4. Caliber compasses .
    See Calipers.
    Caliber rule ,
    a gunner's calipers, an instrument having two scales arranged to determine a ball's weight from its diameter, and conversely.
    A ship's caliber ,
    the weight of her armament.