Unlike rifles and handguns, shotguns are generally measured by gauge rather than caliber. Gauge is determined by the number of lead balls of size equal to the exact diameter of the bore that it takes to weigh one pound. For example, it takes 12 lead balls the same diameter as a 12-gauge shotgun bore to weigh one pound.
The only shotgun that is classified in caliber is the four-ten. The four-ten is designated as .410 and is a true 41 caliber. This designation also is seen in rifles and handguns.
Notice that the lower the gauge number, the larger the bore diameter. Common shotgun gauges are 10-gauge, 12-gauge, 16-gauge, 20-gauge and 28-gauge.