Shotguns gauge

The gauge refers indirectly to the bore or inside diameter of a shotgun. Gauge is calculated as a number of lead balls of a bore diameter required to make up one pound. 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.

For example, a 12 gauge shotgun has a bore diameter such that 12 lead balls of the bore diameter would weigh a pound. This is an old British measurement system. A 12-gauge firearm uses only 12-gauge shells.

The only exception to this is the .410 caliber shotgun which has an actual bore diameter of 410/1000ths of an inch. A .410 shotgun is approximately a 67-1/2 gauge.

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