IF you FIND a machinegun, you may inquire of the NFA Branch of the BATF to determine IF the deceased "Family member" every registered it and just "LOST" the paperwork.
IF it is in the registry, they will tell you and explain the TAX FREE transfer for the Estate to a relative.
IF it was never registered, be prepared to surrender it. That simple !
You may be able to surrender it to a local PD or Sheriff's Office ...who would appreciate the gift ! They can register it to the AGENCY and keep it for law enforcement purposes. It becomes a non-transferrable LEO dead machinegun.