Yes, rifled shotgun barrels are not uncommon for slugs.
But the ATF still treats them as shotguns, at least if the bore is over .50. Note that with the .410 it is ATF Calvin Ball. Saw off a single shot .410 shotgun and go to Federal prison; however, you can purchase a brand new Taurus Judge pistol designed to shoot .410 shells.
Yeah, or a single shot TC Contender with a .410 barrel.