Just a quick hijack, please. I thought we called it a .410 "bore" because it is not a .410 "gauge." A quick search shows plenty of shotshell manufacturers referring to it as the .410 "bore," as well.
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"Bore" and "guage" are interchangableand part of a historical naming convention that was supposedly agreed to via SAMII. It refers to the number of round shot the diameter of the barrel needed to equal one pound. Thus a 4-bore/4-guage needs 4 round shot the diameter of its barrel to equal one pound, while a 12-guage/12-bore needs 12 shot the diameter of its barrel. Different diameter shot for each bore/guage.
A .410-bore/.410-guage shotgun would need just 410/1000th of one round shot the diameter of its barrel to equal a pound and thus would be big enough to be classified by BATFEIEIO *spit* as a Destructive Device. You would need a $200 tax stamp for the firearm, a $200 tax stamp for every round, and possibly orthopedic surgery if you fired it from the shoulder. (It would be just a bit bigger than a 2-bore/2-guage shotgun. The only ones that big that I know of were classified as "punt guns" [go look it up].)
That shotshell manufacturers do not follow the rules they helped set up regarding how to classify shotguns does not surprise me one bit. Why should they be any different than anybody else?
A .410 caliber is nominally .410 inches across. There is a "convention" about just what to call each caliber due to the fact that some of them would end up with quite unwieldy names or very (to us who are used to the current names) confusing names. Just think of what G-d's own caliber would be if we went by the actual diameter of thebullet.
stay safe.
