what ''gauge'' describes in a shotgun. Example: 12, 20, 410.

Shotgun gauges r determined by the number of lead balls of a given diameter required to make one pound of that size ball. Thus 10 balls of 10 gauge diameter r required to make one pound of such balls, or 20 balls of 20 gauge diameter r required to make one pound, & so forth. This is the traditional, & very old, system. The actual (nominal) bore diameters of the various gauges r as follows: 10 gauge = .775 inch, 12 gauge = .729 inch, 16 gauge = .662 inch, 20 gauge = .615 inch, 28 gauge = .550 inch. The .410 is named for its nominal bore size, & is not a gauge at all.

Shot gun gauge refers to the inside diameter of the barrel. Except .410, which is technically a calibur (the inside diameter of a .410 is .41 inches)

10 gauge = .775 inch
12 gauge = .729 inch
16 gauge = .662 inch
20 gauge = .615 inch
28 gauge = .550 inch.

The most commonly used gauges are 12 and 20.