When looking for a new holster, the weapon caliber can be critical to getting the proper fit. The definition of caliber as it pertains to ammunition is the nominal internal bore diameter of the barrel. It is measured in inches or millimeters, depending on the cartridge.
The term also applies to the diameter of a bullet or projectile as discharged from the barrel. Most firearms list the caliber on the top or side of the barrel, as seen when the slide is down. Others may also show this information on the semi-auto slide and typically on the barrel for revolvers.