Bore Size Table
Gauge Bore Diameter or Calibre
1 1.669"
2 1.325"
3 1.157"
4 1.052"
5 .976"
6 .919"
7 .873"
8 .835"
9 .802"
10 .775"
11 .751"
12 .729"
13 .700"
14 .693"
15 .677"
16 .662"
17 .650"
18 .637"
19 .626"
20 .615"
21 .605"
22 .596"
23 .587"
24 .579"
25 .571"
26 .563"
27 .556"
28 .550"
29 .543"
30 .537"
31 .531"
32 .526"
.410 .410"

Gauge - System of measurement for the internal bore diameter of a smooth-bore firearm based on the diameter of each of that number of spherical lead balls whose total weight is one pound. The internal diameter of a 12 gauge shotgun barrel is therefore equal to the diameter of a lead ball weighing 1/12 pound, which happens to be .729" (Or in British: Bore.)

.410 is not really a gauge, except by convention. It is the actual diameter of the bore in decimal inches.

The Gauge/Bore system is also used to describe the internal barrel diameter of large-bore, 19th century, English, single-shot and double-barrel black powder rifles.

https://www.hallowellco.com/bore_size_chart.htm