My curiosity arises from what might happen upon my passing, and if my wife would want to sell the NFA firearms in the trust.
When you establish a trust, you name a trust administrator and a contingent administrator. The administrator is in charge of the trust, can acquire and/or dispose of trust properties. When the administrator dies, the contingent administrator assumes full trust administrator authority and can acquire or dispose of trust property. Ask your attorney, they will have the answer to your questions.