gun

Caliber is the approximate diameter of the circle formed by the tops of the lands of a rifled barrel expressed in hundredths or thousandths of an inch (.308 caliber) or millimeters (7mm caliber).

ammunition

Caliber is a numerical term added to a cartridge name to indicate the approximate diameter of the bearing surface of the bullet.

aka:

• cal (abbreviation)

 

Footnotes / Sources:

“Glossary ⇒.” Sporting Arms and Ammunition Manufacturers’ Institute. Ed. SAAMI’s Technical Committee. Web. 15-18 Jan. 2011.