This is an idea that started brewing in my head quite a while ago, and I'm bringing it to light now.....
Ok, take your average single action revolver. Where the pin holds the cylinder in, replace that with a rod, and redesign the rear of the frame so it goes all the way through to the hammer. At the front of the rod, run a tube along the bottom of the cylinder and port at the end. Viola, automatic six shooter.
Cock, pull trigger. Gas acts on rod, shoves hammer back. You'd better be using a fannable action though, since that's what's going on... it's an auto-fanning hammer, the rod knocks the hammer back, rotating the cylinder, it locks, finger's still on the trigger so the hammer falls, there's the next gas puff and the cycle continues a total of 5 times. Yes, 5. Remember, you didn't use it for the first shot :)
With a little forethought you could have a disconnecter in there, I bet... and then you have the gas-op equivalent of a mateba in an almost correct looking package, except for the extra "ejector rod" under the barrel.