This is an elastic band powered catapult. It can fire small projectiles. It is made in sketchup, and I have no idea how big or small it is, so if you want to make it you may have to scale it yourself. The angle at which the catapult stops, and therefore the angle of launch is adjusted by where you put the small pole.

Once printed, put the poles in the holes that they fit into. If some holes are too small, simply make them the right size with a file. The catapult head probably needs to be glued on to the catapult arm, and you should probably bash something into the ends of the thickest pole that has the arm in it. All you need after that is an elastic band, which you loop round the 'C' shaped holes, and around the arm, and you're ready to go!