You don't need an address to set up a trust--it's a legal document, kinda like incorporating yourself. You can set up the trust in any state you want. Let's say, for instance, if NFA items cannot be purchased in your state that you set up your trust in Texas (where many of the silencers are made). You can then purchase an item for the trust in the state your trust is set up in, go pick it up, and take it home with you. Doesn't matter if you have a stable address or not--the trust has legal status in a state, and as a trust beneficiary (in your case, the executor of the trust) you can take it out of the "trust" (not a physical place) and carry it with you wherever you go. No need to get your grandpa involved at all!

Of course, it's even easier if your state has lax gun laws. Just set it up in your state, have the item shipped to your FFL, have hime transfer it to your trust, and you are golden.

Side benefits: no finger prints, pictures, or background checks need to be done since it's a legal entity, not a person; takes about 16 weeks to get the stamp; and anyone who is named a trust beneficiary can use the items in the trust, including NFA items. So your wife, son, father, daughter, or whomever is a trust beneficiary can use the NFA item WITHOUT you being directly with them.

Gun control laws: ity's time to make them work for our benefit