This is a pretty simple design, allowing for 1 rubber band to be shot at a time. What I've posted is just the mechanism [all the moving parts].
Step 1: Mechanism
Its pretty simple, allowing for 1 shot at a time. It includes the trigger and hook for the rubber band. Just attach this to any strong barrel and it will work... remember to include a place to attach the front of the rubber band [but thats pretty obvious].
Step 3: Hook
Rubber band goes here.
Step 8: Trigger Hinge [same As Step 4 and 5]
do what you did in steps 4 and 5 for this [sorry i couldn't provide a different picture]
Step 9: Frame
For the next few steps you will only need one of these
Step 10: Trigger in Frame
Step 12: Finish Frame
Attach the second frame piece
Step 13: Add Other Gun Parts
This isnt specific, just make a barrel and a handle or whatever. Just make sure its strong.