How do you tell the gauge of a shot gun?

The gauge of a shotgun is determined by the diameter of its bore, specifically the number of lead balls of a specific diameter that would weigh one pound. For example, a 12-gauge shotgun means that 12 lead balls with a diameter equal to the shotgun's bore would weigh one pound. To find the gauge, you can also measure the bore's diameter in inches and then refer to a gauge chart, as the number of balls that fit into a pound decreases with increasing gauge number (i.e., 12-gauge is larger than 20-gauge).