The gauge of a shotgun is determined by the bore diameter
Bore Diameter in Inches
Bore Diameter in mm
• 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:
= (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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