Check out the Saiga-12 shotgun. It isn't full auto, but it is legal, and at $500 to $700, such a deal it, I say.
And it is basically an AK-47. More reliable than which is hard to find.
Do remember tha right now the prices are artificially high due to no importer. But the factory is going to import themselves. Then the price should drop back to about $250.
It isn't stainless, but it is a combat proven design. And cheap!!
PS It popints like a dream - its cheap plastic stock, notwithstanding.
Oops! Should be point, not popints.