Some repeating crossbows

 

And for the crossbow fans among us....Some very innovative designs, including one that shoots a magazine of 45cal. balls, and an electric cocking repeater that uses crossbow bolts. Very impressive engineering.!
Duckman's Website in the UK

The full auto slingshot

Another example of fine German engineering. When I was a kid, the best I could come up with was a high capacity rubber band pistol.   "Weaponizing Black & Decker tools"  A fine winter woodworking project!