Caliber /(?)/

Cal·i·ber

Caliber

n.
  1. 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. (Gunnery)
    The caliber of empty tubes.
    — Reid.
    A battery composed of three guns of small caliber.
  2. The diameter of round or cylindrical body, as of a bullet or column.
  3. Fig.: Capacity or compass of mind.

Phrases & Compounds
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.