I guess necessity was the Mother of invention, but I think one could do just as well with a speedfeed stock or a sidesaddle. But I was also led to a video on the Cosmi after viewing the original video, which I think is a pretty interesting shotgun. Not $17K interesting, but interesting nonetheless.
If a man I once knew could have shown the inventor how fast he could shoot/reload/shoot a single barrel 12 ga. that thing would never have been built. We drove deer with dogs in the swamps when I was a young man and there was an old guy who only wore bib overalls (not camo) and shot a single shot. He held two or three loads of buckshot between the fingers of his left hand when he shot at deer. He could eject an empty, reload and shoot again faster than you could imagine. When hearing the reports lots of people thought he was shooting a pump.
As a small boy I used a Winchester 37A 12 ga.. With it I would have the second shell in my left hand. I could open and reload it, cock the hammer, mount it for the second shot faster than my brother could pump his gun. Spent a lot of time practicing to get fast so I am sure your friend was able to shoot faster than most people could. I gave it up when I was given a Winchester Model 12, 28 ga. for my Christmas present when I was 12. Best Christmas present I ever got.