so, here it is, designed and made in 2 minutes flat!
Pros: simple, easy to make, cheap and mostly easy to come by parts.
Cons: low power, no trigger and it's just a open tube.
WARNING!: may damage rubber band!
Step 1: You Will Need
You will need: an Alan Key, 2 washers, 2 long angle beams, a large rubber band, 3 nuts, 1 short screw, 1 long screw, a pulley, and a spanner.
Step 2: The Tube
Firstly, line up the angle beams, then put the screws through at each end, next secure with a nut on each screws.
Step 3: Securing the Rubber Band
Now add a washer to the long screw, add the rubber band to that same screw, add another washer to the screw, after that add another nut on and tighten.
Step 4: To Load and Fire
To load: turn gun up, put the pulley on the tube in the gun, put rubber band around it, put finger on pulley, draw finger back, and finally to fire lift finger.