This is a much better option than modifying a Tac-14 or Shockwave like some have suggested. That’s $450 for the shotgun, $200 for a SBS stamp (rather than $5 for a AOW stamp), $75 or so for furniture, and $450+ in machining work.
You’d have to cut, re-crown and rebead the barrel. You’d also have to cut + rethread the mag tube and then rejoin the mag tube to the barrel.