This is true, what's missing is that the wedge must have something solid on both sides to push(wedge) against so that it can tighten up. If the arbor is short, the only thing holding the barrel side firm for the wedge to push against and lock is the front of the cylinder, which needs to move. So the arbor is in effect, loose, it's not doing much. When fired, the barrel assembly won't be pushed forward and stay there, it will rattle back and forth. The cylinder should be "floating" between the barrel and standing breech, which should be locked together into a solid, it's not a supporting member.