I'm leaning toward agreeing with Wildalaska, but for arguement's sake I'll ask if your friend's grandfather is still alive? If not, is the grandmother still living? Surely someone in the family will know if it was registered as a bringback or during the amnesty. Try asking some of the close family members before you ask the feds. If you don't get an answer, you can follow the advise of some of the other posters and ask BATFE to look up the serial # in the registry. If it turns out that the MG is not registered, I'd contact an attorney versed in NFA regulation and firearm law in general. Whatever you do, PLEASE do not destroy the gun! Even if it is not legal, there are proper channels to go through that would not lead to the destruction of that kind of historical artifact.