Q: Can you help settle a debate among friends regarding the fact that .410 is a shotgun gauge size?
A: I’m sorry to disagree, but the .410-bore shotgun is indeed a caliber, not a gauge. To quote the NRA Firearms Source Book, “Gauge: A term used in the identification of most shotgun bores (with the exception of the .410 bore).” Shotgun gauges were originally determined by the equal number of balls cast from 1 lb. of lead that would pass through the barrel. Hence, the 16 gauge would allow 16 balls weighing 1-oz. each to pass through the bore; the 12 gauge, 12 balls (of greater weight); etc., and, in fact, the .410 bore is equivalent to a 67 gauge. The use of gauge indication stops with the 36-ga. shotgun. It is true that many refer to the .410 bore as a “410 gauge,” but it is the wrong term.
--John M. Taylor