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 bore is measured and whether the finished bore matches that specification. It is measured in inches or in millimeters. In the United States it is expressed in hundredths of an inch; in the United Kingdom in thousandths; and elsewhere in millimeters. For example, a US "45 caliber" firearm has a barrel diameter of roughly 0.45 inches (11.43mm). Barrel diameters can also be expressed using metric dimensions. For example, a "9 mm pistol" has a barrel diameter of about 9 millimeters. Since metric and US customary units do not convert evenly at this scale, metric conversions of caliber measured in decimal inches are typically approximations of the precise specifications in non-metric units, and vice versa.

ഒരു തോക്കിന്റെ ബാരലിന്റെ ഉൾവ്യാസത്തെ പറയുന്ന പേരാണ് കാലിബർ. ആ തോക്കിൽ ഉപയോഗിക്കുന്ന വെടിയുണ്ടയുടെ ബാഹ്യവ്യാസം എന്നു പറഞ്ഞാലും ശരിയാകും.