yep, rifled barrel over 50 caliber is still a regulated firearm-
Now, a 45-120 would handle a 3" 410 slug but if made to modern specs you could get more out of a 400 or 500 grain bullet than you could out of the shotgun slug. BFG has a 45-70 revolver that I would not fire except in suicide. I think that one would kill at both ends anyway.