A Shotgun's Gauge Sizes shown are the minimum inside bore
diameter with a tolerance of +0.020". Data is presented courtesy
of SAAMI. 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
approximate 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-bore 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