Now that NASA's funding has been tossed up in the air again, advocates of quirkier launch techniques are once again coming out of the woodwork. The space cannon concept has been around a very long time. I think the last time I heard anything about it the main stumbling block was acceleration. It's a big freaking gun, and that means the forces are much higher than a conventional chemical rocket. Engineers weren't certain regular satellites could survive the stress, let alone a bunch of meat bags.