Final answer:
Shotgun shells are measured by their gauge, with the term reflecting the gun's bore diameter and inversely related to the bore size. Unlike rifle calibers which use hundredths of an inch or millimeters, shotguns use an older system based on the weight of lead balls fitting the bore.
Step-by-step explanation:
Shotgun shells are generally measured by gauge. The term gauge is a measurement of the shotgun's bore diameter and represents the number of lead balls of the gun's bore diameter that would weigh a pound. Unlike calibers, which are used to refer to the diameter of the barrel in rifles and handguns usually in hundredths of an inch or millimeters, shotgun gauges are a bit more archaic. For example, a 12-gauge shotgun has a bore diameter that would correspond to a lead ball weighing 1/12th of a pound. The smaller the gauge number, the larger the shotgun's bore. When it comes to shotgun shells, grains are not the primary measurement; grains are typically used to measure the weight of bullets and the amount of gunpowder in cartridges for firearms.