Firearms are treated as personal property and divided amongst heirs equally or as dictated by a legal will, provided the person/s receiving them have no laws preventing them from possessing such firearms.
Handguns are the same, but the executor of the estate signs as the "seller". Handguns crossing state lines must be transferred via an FFL.
NFA items are transferred directly from the estate to the heir/s tax free via the Form 5, provided the heir/s are legally allowed to receive such firearms, this should all be completed before the close of probate. They can also be transferred to a person who is not an heir for the purpose of liquidation using a standard Form 4 and $200 tax.
NFA items being transferred across state lines to an heir via the Form 5 can do so directly, without an intermediate. Otherwise they must transfer to a dealer in that state first.