This appears to be somewhat a matter of terminology misunderstanding. I would not call a gun with a " cone a stepped cone, just a short one. Some old guns have been encountered which had np Cone, just a step which is what I was referring to.
I fully realize we are talking here of guns built prior to the existence of SAAMI but after y were established the recommended a max cone angle, from the horizontal of 5. With chamber & bore both of "Nominal" size this would give a cone length in a 12 gauge of .400". "IF" the bore were .020" oversize which is not uncommon for those older Waterfowlering guns built for use with brass shells the cone length would be reduced to just a few thousandths longer than ". These measurements are not extremely Easy for most to take but with exact measurements of chamber end, bore diameter & either the angle of cone or cone length the other dimension is easy to calculate with a bit of Trig.
