I as tinkering around and made a semi-auto one.
Oh and.. I know it's weak, because I made it mostly for scientific design, not for performance. So feel free to take it farther and make a real shooter out of it. : )
Step 1: hopper and front
this is the hopper and front of the gun. like the last one, the gray connectors and blue spacers are the hopper ;and the black piece under it is there to stop bullets from falling through.