Standard Modern Shotgun Gauges
-- by Rob Ramsdale --

So how did we start calling shotguns 10 gauges or 12 gauges? The nomenclature has been around a long time and it is based on a simple formula. The gauge number is determined by the number of solid spheres of a diameter equal to the inside diameter of the barrel that could be made from a pound of lead. For example, if you take 12 lead balls that have the same diameter as the bore size of a 12 gauge, their combined weight would be 1 lb. Therefore, it would take 20 balls of 20 gauge diameter to make one pound, and so forth. The .410 is not actually a gauge at all but instead a measurement of its nominal bore size. In essence, it is basically like a caliber for a rifle. If it were a gauge, it would be listed as a 67 ga.

As time has gone by, many of the gauges fell out of favor and were gradually eliminated. Partly because it was much easier for the shell manufacturers to make shells for a few gauges instead of a dozen. Nowadays, by far the two most common gauges are 12 and 20. One of my favorites, the 16 ga., is holding on as an upland gun especially. The 10 ga. and the .410 are still fairly popular and the 28 ga. sees some time in the field as well. The rest of them are essentially history as far as shooting goes since you can no longer buy shells for them.

Nominal Bore Sizes Of The Various Shotgun Gauges
Gauge	Inches
.410 (or 67 ga.)	0.410
32	0.526
28	0.550
24	0.580
20	0.615
16	0.662
14	0.693
13	0.710
12	0.729
11	0.751
10	0.775
8	0.835
4	1.052