I had the idea of taking my mini crossbow, cutting off the tips of the ends of the bow, then inverting them in a smaller internal chamber with a wire that gets pulled back when you cock the gun. There are probably already guns like this but I am not sure. It would be like a mini dart gun that could take bb's or airsoft bb's or whatever you drop down the end. How good of a FPS can you get with something like this? What are the best type of rubber/metal string/etc that can be used to attach to the mini crossbow structure to propel the projectile?