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.
