The Colt Root 1855 Revolving rifle was the first weapon issued to Berdan's Sharpshooters. I'm thinking that the gas can be tapped to an operating rod to push the hammer (and rotate the cylinder) back.
A fella' named Stoner had the same thought one night in a farmhouse sometime in the 40s.
BORDERLANDS: THE GRAPHIC NOVEL
A story about a man, his harrowing journey, a vast frontier, and a great struggle.