Take a .357 magnum case and set it up so that it can fire two bullets, not simultaneously, but one with each strike of the primer. So if someone has a 6-shot .357 Magnum revolver, it can now fire 12 shots without reloading.
How the hell would this work? the cartridge would be loaded in layers, from front to back, there would be the first bullet, propellant, an electric primer, then another bullet, another layer of propellant, and in the primer pocket a two stage device. The side of the device facing the propellant would be another electric primer, on the back of the device would be a piezoelectric generator. Somehow an electric circuit would have to be run from the generator to the front primer. And the generator would have a switch in it that would send the current from the first strike to the front primer, and the second strike tothe back primer. The current output from striking the piezo generator would have to be sufficient to ignite the electric primers.
Could this be made to work? Who knows. How good could the ballistics of each shot be, given that case capacity constraints would limit the amount of propellant per shot. How expensive would the be? Probably a few bucks a round.
Of course, the correct response to this should be "Just buy a 12 shot (or higher) 9mm auto. And that would not really b wrong. But I bet there are a lot of people who on 357 revolvers, especially small ones, that might be willing to trade power for capacity, especially if they can do it without changing guns.
Now I know this is kind of nuts, and may be impractical. But if it could be made to work, it would be .357 magnum carriers some interesting options.