How about stove enamelling ?? I had quite a few car components stove enamelled and it is hard as nails. Nice satin black finish, and I believe it can be sprayed on as well as dipped, so you could get them to mask off all the bits you wanted leaving un-coated, and it is quite a thin coating. It was fairly cheap too... probably cost about 15 - 20 quid to get a gun cylinder and barrel done. HTH.