I found a wooden gun like this on a market in The Netherlands, it turned out the shooting mechanism was well documented in an (expired) patent. The drawings from the patent were cleaned up in Photoshop, then traced in Inkscape and assembled in Blender. After test fitting (see picture) I adjusted the mechanism a bit to make it run smoother. The gun body is an exact copy from that of the patent but was pimped up a bit with cool hand grip stock and a safety switch. The video shows that shooting the gun results in rapid firing of the rubber bands. This is a great toy for boys.