What parts can you NOT use charcoal blue on (apart from springs?). I was going to do the trigger and breech block on a Stevens 44 and then is struck me that the trigger is s sear and the breech block is probably heat treated. And I would also exclude screws as fire blue is correct for these (and easy to do), keeps the slots nice and hard. How about Mauser small parts, like cocking piece, bolt shroud ... these are hardenec parts as well, and they will soften up some at 850 (which is what I set my furnace to for CB)