Perhaps you are referring to the revolvers like the Russian Nagant. As the round is fired, the cartridge case expands and seals the gap between the cylinder and barrel. The cylinder is drawn back away from the barrel as the cylinder is cycled for the next round, which allows the protruding casing to clear the barrel, it then moves forward as the chamber is aligned for the next round. Incidentally, it's the only type of revolver that can be effectively silenced.

I hope that's what you were referring to,
Good luck, Steve