Unlike rifles, shotguns are generally measured by gauge rather than caliber. The term gauge is a unit of measurement used to determine the number of lead balls 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.
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.