OK- listen up, pay attention. There has been some bad information posted here, and this is a very serious matter. Are you taking notes?
In the case of a fully automatic firearm- that is located in the US- to be legal to possess the gun OR the receiver of the gun, it must have been registered with the BATFE prior to May 1986.
If someone else registered it, you now possess it, and you have not transferred it to your name, you have a problem.
WAY back when, there were DEWATS- deactivated War Trophies- and paperwork for them. That is now worthless. DEWATS is no longer recognized.
There are two ways of legally demilling a machine gun. 1. Total melting 2. Flame cutting (torch) the receiver into 3 or more pieces. At one time they were being cut with a saw- too many people rewelding.
If the guns WERE registered with the BATFE, the paperwork may be something like THIS:
and will have a stamp like a postage stamp on it- for $200.
Guns that WERE registered can be transferred when inherited- no $200 tax, you must be in a state where it is legal to own.