Attorney David Goldman recently posted an article, Free NFA Trust Form for Class 3, Title II purchases, about a dealer who was offering a free NFA trusts with purchase of a NFA firearm. As Attorney Goldman discusses in his article, the trust was blatantly invalid. The invalidity of the trust could subject the trust assets to forfeiture and the grantor/trustee(s) subject to violating the NFA with penalties of up to 10 years and $250,000. Here is a copy of the Invalid NFA Trust. Because attorney Goldman has done such a great job at blogging about this issue, I would refer you to his post (see link above) for a more in-depth review.
This really underscores the need for individuals to have PROPERlegal advice when setting up a trust.