A Shotgun's Gauge
Shotguns are measured in gauge. For shotguns, gauge is the number of lead balls, each the diameter of the gun’s bore, that it would take to weigh one pound. For example, if you have lead balls the same diameter as a 12-gauge shotgun bore, it will take 12 of those balls to make one pound.
As shown in the gauge size diagram below, the larger the gauge number, the smaller the bore diameter. For example, a 20-gauge shotgun has a smaller bore diameter than a 10-gauge shotgun. It takes more balls, of a smaller size, to equal one pound.
The .410 is the only exception to the way in which shotguns are measured. Look carefully at how this number is written. It is actually a caliber because it has a decimal point (.) in front of the number! The .410—if measured in gauge—would really be about a 67 gauge!