The gauge of a shotgun is determined by the bore diameter
Shotgun Gauge
Bore Diameter in Inches
Bore Diameter in mm
410
0.410
10.4
28
0.550
14.0
20
0.615
15.6
16
0.670
17.0
12
0.730
18.5
10
0.775
19.6
where:
• The 410 gauge is not a true gauge.
• A shotgun's gauge is based on the gauge of a smoothbore musket. This was defined on the number of lead balls fitting the barrel that would weigh 1 pound.
If this data is analyzed in Minitab:
gauge =
= (215.7 * ((bore diameter in inches)^2)) - (364.5 * (bore diameter in inches)) + 163.1
Choke is an intended constriction in the bore at the muzzle. A measurement at the muzzle of a shotgun with any constriction could give a misleading gauge estimate, so care should be taken to take the bore readings at several points.
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