Gauge is the unit of measurement for firearms (typically shotguns) when describing the inner diameter of the barrel.
To determine the gauge, you must use a reciprocal equation to determine the weight of solid spherical lead balls which can fit into the barrel. This means the number of lead spheres of equal size compared to the approximate diameter of the bore that it takes to weigh 1 pound.
Here’s an example for a 12-gauge shotgun:
You would need 12 lead balls (each with exactly alike diameter to the 12-gauge shotgun barrel bore) to weigh one pound.
The original terminology for gauge came from the use of cannons when figuring out how much cannon ball can be backed into a cannon.
Common Shotgun Gauges:
Remember, these sizes operate in a reciprocal manner (meaning inverse) so the smaller the gauge number the larger the bore of the barrel.
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