You could try loading it with snap caps and dry firing the crap out of it. If yours is put together the way I remember a friend's, it's held together by rivets and doesn't lend itself to disassembly. You could try squirting some kind of mild abrasive (like toothpaste) into the lockwork to speed up the smoothing process while dry-firing. I'm thinking here or the stamped steel models that they sell chambered in .45LC/.410.
If you have one of the ones that's a replica of the old Remington derringer, they are made out of crap metal and there isn't much that can be done with them.