Shotgun gauge refers to the diameter of the bore, or, more simply, the inside diameter of the barrel.
The common gauges for upland hunting and clays include, in descending order of size, 12, 16, 20, 28, and .410. The 12 gauge gun is likely the most common shotgun in existence. It is versatile, practical, and appropriate for hunting virtually any species of North American upland game, as well as participation in sporting clays, trap, and skeet. But what does the ‘12’ in 12 gauge refer to?