That chamber would definitely have been intended for use with paper cases.
Note in this context I refer to "Brass Cases" as those thin walled all brass cases. The paper cases with full length brass shells I refer to as a paper case. The brass was there for the purpose of containing swelling under wet conditions as waterfowling etc. I believe also the paper normally extended past the brass for a length sufficient for crimping, so upon firing the paper actually met the cone.
The idea behind this sharp step was that the case would open against the end of chamber & provide a non-interupted transition from shell to bore. In practise the case was not usually as thick as was the step so was still a sharp corner to cause ahot deformation, plus shell cases were not held to a consistent enough lenght & some fell short & some overlapped. Again I am quite surprised a chamber of this type being found from a gun as late as 1880 from as respected a maker as WR.
