For shotguns what does gauge measure?

The gauge of a shotgun is the measure of its bore diameter. The larger the gauge, the smaller the bore diameter. For example, a 12-gauge shotgun has a bore diameter of approximately 0.729 inches, while a 20-gauge shotgun has a bore diameter of approximately 0.615 inches.

Shotgun gauges are traditionally measured in terms of the number of lead balls of equal diameter that would fit into one pound of lead. So, a 12-gauge shotgun would require 12 lead balls of equal diameter to weigh one pound, while a 20-gauge shotgun would require 20 lead balls of equal diameter to weigh one pound.

Bore diameter is not the only factor that affects the power of a shotgun. The length of the barrel, the amount of choke (constriction at the end of the barrel), and the type of ammunition used all play a role in determining how much energy the shotgun produces.