W.R.Buchanan - Look up "the 12ga from Hell" thread, IIRC it's maybe started on the AR site but it is ALL over the web - Those are just about what you are thinking of maybe? It's even on the Zombie Squad forums. That could do you nicely here; I'm not sure I want one (well, of COURSE I want one, but more "if I dare get one and mess my shoulder up" probably!) - Could load it gentle though if needed, and maybe make it to use cut down 50BMG brass so you'd have to do some extractor work or ?? For sure could use brass 12 Ga shells.
https://thefiringline.com/forums/sho...d.php?t=197214