What the OP wants to buy is a Saiga 12 with a drum magazine. In my opinion it's hands down the most "powerful" and destructive civilian legal firearm available in the US. You can spray 12 gauge shotgun shells down range as fast as you can pull the trigger. I don't think it would be too hard to bring down an entire single family home with one of those things by emptying a few mags into the support beams/foundation. It's definitely what I would use in a zombie apocalypse. Full auto guns aren't legal without lot's of paperwork and licenses so I wouldn't necessarily count those as "legal".