What does 'gauge' refer to when discussing shotguns?

The term 'gauge' when discussing shotguns specifically refers to a measurement of shotgun bores. It is used to describe the internal diameter of the shotgun barrel and is critical in determining the type of ammunition that can be safely and effectively used with that shotgun. The gauge number reflects the number of lead balls of the bore's diameter that it would take to equal one pound. For example, a 12-gauge shotgun has a bore diameter such that 12 lead balls of that diameter weigh one pound.

Understanding gauge is essential for safe shot shell selection; using the wrong gauge can result in unsafe conditions and can damage the firearm or lead to accidents. This is particularly distinct from the other choices, which either relate to different aspects of ammunition or are not relevant to the measurement of a shotgun's bore. For example, cartridge power relates more to the velocity and energy of a cartridge rather than its dimensions, while counting powder pertains to reloading practices. Similarly, terminology for the type of shot used does not pertain to the measurement of the shotgun bore itself.