Any items owned individually can transfer to your heirs tax free on a form 5, after your death.IANAL: The trust can have anything on it, not just NFA items. I understand the flaw inherent to it since there's no checks and balances over who is added after the fact, but that doesn't automatically give the ATF power over the trust. Pertaining to the members, my trust is bound by the conditions set forth within the trust. It is a legally binding document that the ATF can't just change. However, they can change the rule that all members of the trust must submit for new approvals. For mine, changing the members is an amendment. The ATF reviewer can see if I routinely remove and add members for the purposes of approvals.
I also just watched a video from MAC stating all his NFA items were done as an individual. I assume he's got quite a collection at this point, so when he passes, the value of that collection is lost to his heirs unless they want to pay $200 per item. This whole transfer tax is a racket.