Generally when creating a NFA trust, one must look to the laws of their state and how they affect the right to own a class III firearm.
What happens if you live in New York or any state where some Class III firearms are banned but plan to use and keep them in another state where they are legal? Can you a resident of a state where the item is banned purchase, store, and use the items in a state where its legal.
According to the ATF, an individual can purchase an item restricted by the NFA that is not permitted in the state of residence of the trustee, when the trust will be located, and item will be only used and maintained in states where it is legal.
This means a NY resident can purchase a class III firearm in PA when it is under a PA trust and the firearm is kept and used in PA.