Oh geez, the ancient question comes to me again. I've been looking for the right grinder for dry gutloads for some time now. SPF uses a Ninja, the 1000 watt 6-bladed blender from the fiery pits of hell. (sorry, couldn't resist)
It does an ok job at grinding the beans and hay we use in our Cricket Crunch gutload formula, but we still have to sift the final product for little bean pebbles that remain in the mix. To grind these, you'll need either a commercial coffee grinder (which I've heard suggested but personally think would do a substandard job according to its cost) or a stone mill, which I'm thinking is the only correct answer.
Pity that stone mills cost an arm and a leg and need to have their stone changed out periodically.