I have a suppressor on a single-shot trust. I want to add someone else to the trust, and remove myself as the primary trustee. I know I can add him as a supporting trustee, but I don't know if I am able to assign him full rights of the trust and additionally, remove myself.

If I assign him as a supporting trustee, and we go our separate ways in life and he's just indefinitely "borrowing" the suppressor, what happens 50 years from now when one of us dies?

It sounds like you're trying to do this instead of transferring it to the guy with a Form 4, which would be a bad idea as it could constitute tax evasion.

Otherwise, you can usually add and remove trustees, but it will depend how your trust is written and what it allows in it's language.  The wording in the trust would also detail what happens when a trustee/grantor/etc. dies.  Typically the property then goes to the beneficiaries listed in the trust.

Wasn't that the wink-wink advantage of a Single Shot Trust? You restate the trust to change the trustee and the effective ownership of the single NFA firearm in the trust while the legal owner remains the trust so there's no transfer fee or wait.