bcamos,
A specific gun trust has language that can come in handy if a trustee dies.
So it really depends on what you want to add. Look at it from a risk/reward perspective. If you are talking about an AR style SBR, you are only going to have $300 invested in it. ($100 for the lower, $200 for the stamp, remember, everything else is just "parts")
So it doesn't make sense to get a firearms specific trust done for $500. In that case, just add the SBR to the existing trust and be done.
In contrast, if you are going to own machine guns worth thousands of dollars, it would be stupid to not get the best trust you could get.
Remember, its not about the legality of it when everyone is alive. Its about how smoothly things shift if a trustee, particularly the settlor, dies.
I know people who have set up NFA trusts using Quicken Willmaker. It works for the ATF as far as transferring NFA items. But we won't know for a while how it allows assets to slide in the event of a death.
Don