I used the nobby piece from one of my old technic sets. The trigger is pulled down to let the axle spin releasing the rubber band. The bar at the top stops the trigger fromm going any higher.
The trigger is always pulled here to keep the tension on the nobs so the rubber band doesn't fire. the pressure from your finger pulls it down enough to let the rubber band go. after you fire it snaps back and your ready to reload.
The piece at the end holds the rubber band.
A look down the sights.
I am over all happy with the design. The only thing i probably will change would be the color, but thats not a problem when you can still fire a rubber band 20 feet. I hope to make a
semi-automatic gun in the future.