I once (1967) had an acquaintence who brandished a very nice MP-38 when I and a few fellow gun enthusiasts visited him at quitting time at his business to pick up a key to our club range... He was a welder by trade, and evidently kept the gun locked up at his business.
He claimed he had smuggled the gun back in his duffel bag after WW2, and that he "couldn't wait" for their to be another riot in Philadelphia (where I was raised) so he could mow down anyone who came near his house...
To say that the rest of us were shocked by his words would be an understatement... The guy drank a lot and wasn't the type I would pick to hang around with, but he was a club member before I joined and he was the key custodian...
Very mysteriously 
, he was visited at his business in a couple of weeks by local ATF agents who told him they had received an anonymous tip that he possessed it, and to produce the weapon, or they would search for it, and if they found it he would be under arrest.
When he brought it out, they gave him a choice...( because he was a WW2 vet), ...he could surrender the gun, or he could cut it himself multiple times with an acetylene welding torch until they deemed it demilitarized to the point that it could not be reactivated... I can imagine the tears that ran down his face as he welded it up...
Ironically, if he had just kept it hidden and never mentioned it to anyone... he could have registered it less than a year later when the ATF declared the last unregistered machinegun amnesty... in 1968.
I was only sorry for the loss of the historical firearm in excellent condition... not for the owner... Some people shouldn't own firearms... (sigh!)