Rn,
I took that step to be an abrupt and small step at end of chamber, NOT a forcing cone. The diameter just before that step is reported to be 0,711 and ahead of that step it is reported to be 0,709, not much of a step really not a forcing cone and 0,709 is quite large for a 16 gauge bore. Because we don't know ID of bores, we don't know if it is quite overbored or if that 0,709 tapers down to something like 0,665, making a true forcing cone.
I would have considered that taper at last 1/2 cm of chamber to be a curosity if I did not have some old hammer doubles from early 1900s that also will not accept American 12 and 16 gauge chamber gauges to the full 65 or 67 mm depth of the chambers. FYI, all of those guns shoot really nice patterns with card and fiber cushin wads.
Lars
