I've been thinking about this a lot too. First of all, I'm not a lawyer. I didn't even play one on television. And I didn't sleep in a Holiday Inn last night. You should always seek professional legal advice. (I did. It was expensive and worth it.) What I write here is based on my own experience with a revocable trust.
My first question is why would you put your guns into a trust as opposed to putting them in your will?
With a trust, you can put certain limitations on what the trustee can do with the guns.
A revocable trust for your guns will allow you to direct a third party to hold onto and maintain your guns for an heir, if said heir is not old enough to take possession at the time of your death or if said heir doesn't want them.
This differs from a will in that the guns (or any other property for that matter) are preserved for a time in someone's care as opposed to going immediately to your heir. For example, if I just leave my guns to my son, he will simply sell all of them and buy video games. If I leave them to another family member (my cousin in my case) they become his property and leave the possession of my immediate family forever. He leaves them to his sister's kids and that's the end of that.
As far as legal things to protect the trustee? From what do you believe you need to protect the trustee? As long as your guns are all legal to own and the trustee meets all of the requirements to own them, you shouldn't have any issue.
HOWEVER...
If your trustee lives in a state that passes new gun laws after your death, he or she will be legally required to follow those laws, and there will be a legally documented and recorded list of every firearm and serial number. SO...If an assault weapon ban passes that requires turn in/buy back, your trustee will be required to adhere to those laws. In the case of a state ban, your trustee may be be required to either adhere, or relocate the guns to another state. If your state passes storage laws (e.g. guns must be kept in a safe-as we've seen in Australia and England) your trustee will be required to go to the expense of adhering to that law. (It is possible for you to set up a bank account in the name of your trust with some funds specifically for the costs associated with maintaining your guns or adhering to such new laws.)
Conversely, if you simply put your guns into a will, the person who gets them can do as they wish with them, including selling guns rather than complying with the regulations if/when they change.
With a will, you can avoid the creation of a list of your guns by simply leaving "all my guns" to whomever you choose. For example, my dad left me "all his tools" (that was the verbiage in his will). He didn't inventory every single tool he had acquired during his 50-year career in his will. However, if you choose to distribute your guns to numerous people, you'll have to create a list (Jimmy gets my shotgun, Jenny gets my 22 etc etc.)
Lastly, a word about probate. You want to avoid this like the plague. The only people who win in probate are the lawyers. You can avoid this by seeing an attorney who specializes in estate planning and expressing your wish to stay out of probate. They know all the tricks to this, and the secret is having a solid, legal, non-contestable plan for your estate. We did this with my mom before she died. It cost several thousand dollars but it was money well spent.