gauge is the unit of measurement of the diameter of the shotgun bore. The gauge is the number of round lead balls, of the same diameter as the gun bore, required to make one pound. example: a 12-gauge or 12-bore gun has a bore of such diameter that 12 pure lead balls of the diameter the size of the bore would weigh one pound. The 410 is the exception and is called 410-bore because it is .410 inches in diameter...
10 gauge 0.775 inch
12 gauge 0.730 inch
16 gauge 0.670 inch
20 gauge 0.615 inch
410 bore... 0.410 inch
and yes my single shot 410 bore is one of my favorites......has accounted for many racoon