Thanks everyone for taking time to respond. That's quite a variety of responses. It sounds like ownership of such a relic is problematic at best and a legal liability at worst and something that I will avoid.
Question for James K (or anyone else who cares to respond). You said that a weapon can't be torched and demilled in order to make it legal. Is this true? Torching a receiver turns it essentially into scrap and there shouldn't be anything illegal about owning scrap. I've seen torched MGs offered for sale on ebay (I think).