Bingo we have a winner. An NFA item, whether machine gun, SBR, SBS, or any other weapon must at all times be under the "control" of the registered owner. If you are at a range, anyone can hold it or shoot it aslong as you are in the immediate area. Don't go off to get a sandwich and leave it with anyone. Likewise don't leave it with anyone overnight or for any period of time for any reason unless you're in the area. If repairs are necessary you should get a Form 5 approved by BATF with a notation that the weapon will be repaired and returned to you upon completion of repairs. It is also acceptable to send a letter to BATF with the same information, but leaving it with anyone is a transfer of the weapon.
Trusts are a little different. Any listed trustee may possess the item legally.