Couple of things:
First, you don't necessarily need a trust to handle inheritance. It does make it somewhat easier in that you can specify exactly who you want to become trustee after your death, and that person effectively gains control of whatever property the trust has at that point. For example, in my trust I am the sole trustee, but my wife is set up as a successor trustee and my son as the beneficiary. If I die or become incapacitated, my wife becomes a full trustee and has the duty to administer the trust for the benefit of my son. (i.e. until he is old enough to become a full trustee and have possession of the items in the trust.)
If you personally own NFA items (i.e. they are registered to you, not to a trust) your heirs can still inherit them tax free. It just requires ATF paperwork and they'll be administered with the rest of your estate, which may mean probate, etc depending on how you have things set up.
Second, remember that items in a trust do not belong to you and are not your property. They are the property of the trust. Thus, when you die, your will or how your estate is handled has no bearing on what happens to items that are property of the trust. Those will be administered according to what is written in the trust itself. Thus, if you have a trust that owns some NFA firearms, and you specify in your will what is to be done with your firearms when you die, that has zero bearing on what happens to those NFA firearms which belong to the trust, because they are not your legal property.
Very helpful. Thanks!. That last paragraph really put things in perspective. Appreciate your taking the time to reply.