Shotgun Gauges
Gauge
Actual Diameter
10
0.775" (19.7 mm)
12
0.729" (18.5 mm)
16
0.662" (16.8 mm)
20
0.615" (15.6 mm)
28
0.550" (14.0 mm)
Shotgun gauges are 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 are required to make one pound of such balls, or 20 balls of 20 gauge diameter are required to make one pound, and so forth. This is the traditional, and very old, system. The actual (nominal) bore diameters of the various gauges are as shown in the graph (right). The .410 is named for its nominal bore size, and is not a gauge at all.
