The crossbow in the pic is a German stonebow, fitted for shooting bolts. Take off the grooved track and replace the string with a double stone-string with pocket, and you can shoot baked clay balls or metal shot. I think you can find diagrams of this sort of stonebow lock in Egon Harmuth's "Die Armbrust" If not there, it might be in Payne Gallwey's "The Crossbow" Be a bit careful about using PG, though. Sometimes his internal drawings are inaccurate. Anyhow, you need to search for 'stonebows' or 'bullet bows'. Have fun. Geezer.