Shotguns are classified by gauge, which is a

measure related to the diameter of the

smooth shotgun bore and the size of the

cartridge designed for that bore.

 

Common shotgun gauges are 10 gauge, 12 gauge, 16

gauge, 20 gauge, and 28 gauge. The smaller the gauge

number, the larger the shotgun bore. Gauge is determined

by the number of lead balls of size equal to the exact

diameter of the bore that it takes to weigh one pound. For

example, it would take 12 lead balls with the same

diameter as a 12-gauge shotgun bore to weigh one pound.

Today, however, gauge can be measured much the same

way as caliber by measuring the inside bore diameter.

The .410-caliber shotgun is the only exception to the

 gauge designation for shotguns. It has an actual bore

diameter of 410/1000ths of an inch, which is

approximately equivalent to a 67½ gauge.

Each gauge of shotgun shoots only cartridge of the same

gauge. For example, 12-gauge cartridges are used only in

12-gauge guns.

The gauge of a shotgun is usually marked on the rear of

the barrel, and the gauge of a cartridge is marked on the

cartridge as well as on the factory box.