Gauge
The bore-size measurement for shotguns, based on how many lead balls of bore diameter make a pound — lower gauge means a bigger bore.
Shotgun bore size is measured in 'gauge', an old system based on how many round lead balls that just fit the bore would weigh one pound. So 12 balls of 12-gauge diameter weigh a pound; 20 of 20-gauge. Counterintuitively, a smaller gauge number means a larger bore — a 12 gauge is bigger than a 20 gauge. The .410 is the oddball, named by caliber (.410 inch) rather than gauge.
Twelve gauge is the versatile all-around standard for defense, hunting, and sport, with 20 gauge a lighter-recoiling option popular for smaller-framed shooters and upland hunting. Gauge determines shell size and the payload of shot or slug the gun can throw.