I posted a detailed thread about the revolvers in 2018, in the gunsmithing forum. Just did a web search for the gunsmith, Selby S Webb, and lo and behold, the CGN thread turned up as one of the hits.
Years ago, I made a quantity of tapered octagon to round barrels. To index the cuts for the octagon section, I took a piece of octagon barrel and turned a section to fit smoothly into the bore of the barrel blanks. Bored and threaded the piece, with an internal taper at the round end, and cut slits in it. Made a piece that threaded into this bore so that when it was advanced, the cylindrical section would expand, locking it to the bore of the blank. An internal collet. Clamped the protruding octagon at one end of a jig and the round portion to a V-block at the other. Mill a flat, rotate one step, mill another, and so on. Worked fine. That is why I suggested hex stock for the cylinder blank. Accurate indexing built in.