I think you can totally do anything you want to do with a trust, via an LLC.
DISCLAIMER- discuss with a NFA-saavy attorney, mainly to ensure that your LLC agreement has the proper language, whcih should be in any NFA trust (somethign to the end that anyone who is not legally entitled to own or possess firearms is not authorizsed to possess company property that is also a firearm or NFA item) - END DISCLAIMER
Barring future purchases (This is the real problem for switching from Single Member LLC to ANYTHING else) and assuming your NFA-owning LLC isn't also a currently operating business:
Assuming you have a member-managed LLC, you could make a person a member, however this typically means that they own an interest in the LLC. Would probably work great for your kids, not so well for your shooting buddy.
You could revise your operating agreement to make the LLC a Manager-Managed LLC (note that this MAY require filing an amendment with your state's secretary of state office) and appoint a person as a manager, and describe in the LLC agreement that the Manager has the authority to possess company property, but can be removed at any time by a vote of X% of the Members.
OR, you could appoint someone as an authorized agent of the LLC for XYZ purposes, can be removed at any time by a vote of X% of the Members.