OK so this is a little concept I cobbled together over the last couple days. In essence, it is a repeating bullpup slingshot/crossbow. Thereoretically, the magazine pusher could be enclosed, and attached to a bolt, for ease of reloading. Also, the horizontal mags could be made removable, something which few slingshots have. As you can see, this is really just to show that the feeding mechanism works. Which it does.
As for a firing mech, a simple ratchet could be implemented in the top bar/rail, released by a lever system originating at a trigger on the handle at the front. This means a compact bullpup design with all the technical gubbins behind the trigger, embedded in the stock. This leaves more room for stretching the bands, which will likely be attached to a bow in front of the handle.
Anyhow, please leave any feedback in the comments, and I will be sure to update this 'Ible with any progress I make.
(Obligatory notice - my first post: be gentle.)