W W Greener in The Gun & its Development mentioned guns with a sharp step at the chamber end. He also stated the purpose was to do away with the "Jump" across the cone using paper shells. Besides the fat that shells were not held to that precise a length another problem he mentioned was the problem of cleaning the sharp corner at the step, thus leaving a place for corrosion to start.
My understanding of the Chamberless guns were that they used a normal size shell but with a larger bore to accommodate the larger wads of the thin bras shells.
