A Shotgun's Gauge

Shotguns are classified by gauge, which is a measure related to the diameter of the smooth shotgun bore and the size of the shotshell 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.
• 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 shells of the same gauge. For example, 12-gauge shells are used only in 12-gauge guns.
• The gauge of a shotgun and the length of the chamber are usually marked on the rear of the barrel. The gauge of a shell and its length are marked on the shell as well as on the factory box.
• If no length is marked on the barrel, take the shotgun to a qualified gunsmith to have it measured.