I made my carrier block, barrel plates, recoil plate, and cocking mechanism from 17-4 for the only reason that it was the first bar of steel I found at the scrap yard that was 3 inches in dia. and cheap. My bolts were made from bar ends of 303 SS I dug out of the scrap barrel at work. I had some oil-lite bronze bar stock in my toolbox so I turned and bored my own bushings and thrust washers. I say use what is readily available, within reason.