Nice catch!
The Stryker 12 shotgun has a very simple "auto gas eject" system, as I call it. There's a small cutout at the cylinder gap, and a channel, which directs enough gas into the previously fired chamber to eject the previously fired shell. The loading gate keeps the loaded shell in the chamber when you fire the first shot, and then opens as soon as the cylinder rotates to the next position to fire shot number two, and it ejects empty shell number one. That only leaves the last fired shell to eject when you're done with all twelve shots in the cylinder. It works partly because the cylinder is enclosed inside a steel drum.
I don't suppose such a system could be employed to eject spent caps, but it would be handy. Of course some of the gas from any shot is going to pressurize the chamber next to it. If we increase the effect somehow, we might also increase the incidence of a chainfire on the first shot. Then again it is 1860s tech. 21st Century tech already offers solutions.
