Caliber is used to describe the size of a handgun’s bore and the size of the cartridges that will safely fit and fire from the specific bore size. Additionally, caliber is the specified nominal internal diameter of a handgun’s bore and is measured in inches and specified in hundredths of an inch. For example, a “.22 caliber” handgun has a barrel diameter of approximately 0.22 inches, and a “.45 caliber” handgun has a barrel diameter of approximately 0.45 inches. To shoot a “.22 caliber” handgun, you would use “.22 caliber” ammunition, whereas to shoot a “.45 caliber” handgun, you would use “.45 caliber” ammunition.
See examples (not to scale) of the relative sizes of various caliber handguns.