As much as I hate to be a party pooper, and I can easily understand the sentiment behind statements like "I would just shut up and keep it", announcing in a public forum that you would willingly commit a Federal crime, given the opportunity, doesn't seem like a smart idea to me. Yeah, I know, yiou were only kidding, right?
But saying you would keep it, and/or advising the "finder" to keep it is advocating an illegal act. And it's against forum rules to do that, even as a hypothetical.
So, If all you have to post is "I'd keep it" or something similar, adding nothing of value to the discussion , please don't bother. That way the big mean NAZI MODS won't get mad or upset with you. And, yes, I'm one of them.
(please note that the phrase "NAZI MODS" is my sarcastic humor, and I am in no way implying that any of the mods are, or were Nazis, or support any Nazi aims, goals, or philosophy. Its a sarcastic comment on some of the complaints we hear when we remind people of forum rules)
Before you get all bent out of shape about us being censors and stomping on your rights, remember this is a private forum, and we (Mods included) are guests. The rules are the rules, and breaking them gets you punished.
As to "found" machineguns, if they were never registered with the ATF, there is currently no legal way to register them. Posession is a crime, and sadly, even giving one to a museum is still a crime (although a slightly different one). The only legal avenue is to contact, and surrender the gun to the ATF. Period. If the gun has some historical significance, you could try to get the ATF to donate it to a museum, but you cannot, you can only give it to the ATF. You cannot "dewat" it and keep it, either. And before somebody says "take off all the parts and destroy the receiver", that would also be a crime. Sorry.
StrongSide, it would be interesting if you could provide a time frame when your "friend" got help from the ATF and was allowed to register it. As far as I know, it could only have been pre-1986. The registry was closed by the Hughes Amendment to the Firearms Owner Protectection Act in 1986, a Federal Law. Opening the registry (making exceptions to the law) for a nice guy with no criminal intent just doesn't seem like something the ATF would be likely to do these days.
All else being equal (and it almost never is) bigger bullets tend to work better.