Caliber and GaugeThe caliber and gauge are measures of the bore or inside diameters of the barrel used by a particular rifle or shotgun.
The caliber is a direct measure of the inside diameter of a rifle barrel. It is expressed in hundredths of an inch; for example, a .22 caliber rifle has a bore of 22/100 inch in diameter. It can also be given in metric terms such as 5.56 mm.
The gauge refers indirectly to the bore diameter of a shotgun. It is not a direct measure, but is calculated as the number of lead balls of a bore diameter required to make up one pound. This is an old British measurement system. Thus, a 16-gauge shotgun has a bore diameter such that 16 lead balls of the bore diameter would weigh a pound.
Gauge measurement has no connection to the variety of shot sizes that may be found in shotgun ammunition. The one exception to the indirect measurement of gauge is the .410 shotgun. In this case, the measurement refers directly to the actual bore diameter or caliber.