SDC
Member
Just after WW2, there was a French 
barf: ) submachinegun called the "PM9" which used one of the neatest actions I've ever seen; it used a rotating flywheel with a lug in it to drive the bolt forward, and that lug struck a firing-pin in the bolt to fire the cartridge. Since the axis of the bolt was just above the rotational axis of the flywheel, the mechanical advantage of the flywheel kept the action closed until pressures had dropped to a safe level. Since the flywheel was only ~2" across, this meant that the entire submachinegun could be folded up into a package about 13" long. From a mechanical standpoint, that would have to be my choice.
barf: ) submachinegun called the "PM9" which used one of the neatest actions I've ever seen; it used a rotating flywheel with a lug in it to drive the bolt forward, and that lug struck a firing-pin in the bolt to fire the cartridge. Since the axis of the bolt was just above the rotational axis of the flywheel, the mechanical advantage of the flywheel kept the action closed until pressures had dropped to a safe level. Since the flywheel was only ~2" across, this meant that the entire submachinegun could be folded up into a package about 13" long. From a mechanical standpoint, that would have to be my choice.