These sorts of things are not uncommonly recovered by divers wrom shipwrecks and aircraft wrecks and put on display. I've never heard of anyone being hassled. I can think of a dozen aircraft mounted .50 MG's that are in a variety of places including public displays. There is a German 20mm Oerlekon deck-mounted auto-cannon from the submarine U-352 sitting on the dock behind the dive shop that's been seen by literally tens of thousands of people, and it's never been an issue for anyone. "Maybe" it's still an automatic cannon... maybe not. Figuring it out seems to not be a priority for anyone. Personally I would treat it so it does not continue to deteriorate(harder to do than you might realize) and stick it on the wall. That's just me.