In guns, particularly firearms, but not artillery, where a different definition may apply, caliber is the specified nominal internal diameter of the gun barrel bore – regardless of how or where the bo
7.92 mm caliber rifle barrel (Mauser m/96)
Rifle cartridges: from left: 50 BMG • 300 Win Mag • 308 Winchester • 7.62 × 39 mm • 5.56 × 45 mm NATO • 22 LR
A 45 ACP hollowpoint (Federal HST) with two 22 LR cartridges for comparison