Shotgun shells are generally measured by:

Shotgun shells are typically measured by gauge, which is a unit that reflects the size of the shotgun bore. The gauge number indicates the number of spherical lead balls of a specific diameter (equal to the bore diameter) that can be made from one pound of lead. For instance, a 12-gauge shotgun means that twelve lead balls, each the diameter of the bore, weigh one pound. This standardization helps to categorize shotguns and their corresponding ammunition based on the size of the bore, making gauge an essential measurement in the realm of shotguns.

Using calibers, which are commonly employed for rifles and handguns, refers to the diameter of the bullet, generally expressed in inches or millimeters, and is not applicable to shotgun shells. Grains measure weight, primarily used in the context of bullets for firearms rather than shotgun ammunition, which focuses more on the gauge. Therefore, gauge is the correct method for classifying shotgun shells.