If BATF ruled that a combat shotgun, with a bore over 1/2 inch (that's both 12 and 20 gauge, right?) was a destructive device rather than a sporting shotgun, would they be totally wrong?
Look at the definition of "destructive device" under federal law. I think 1968 was the year Congress amended the NFA to include those kind of weapons.
If you have a semi-auto firearm and it uses high capacity magazines (twice the ammo capacity of a typical sporting shotgun) and had other tactical features that were of little or no interest to hunters or clay target shooters, and if the bore diameter is well over .50 caliber...
... would BATF be correctly or incorrectly call it a destructive device?
(P.S. If you tell me that there are shooting games and competitions for "combat" or "tactical" shotguns, I'll reply that there always will be competitions for ANY weapon. Field artillery cannon. Machineguns. If hand grenades were legal and didn't cost so much for civilians (maybe $100 per grendade, plus $200 each for the transfer tax) there would be grenade-tossing competitions too! Just because somebody somewhere invented a game to pay with a weapon doesn't make it a sporting weapon. That was clearly not the intent of Congress.)