My main interest is L.C. Smiths. In Brophy's book "Plans and Specifications" there are factory drawings of chamber lengths and the taper used. Gives the starting bore size and the ending bore size. For a 12 ga. the chamber length from the begining was 2 3/4" or in some 3" if stamped. Start was .811 and 2.750 later waas .797. Forcing cone taper on all gauges was .014 in 2 3/4".
I'm sure other makers had similar drawings.
I've made these plugs in aluminim and they fit right to the 2 3/4" mark in most guns. The guns they don't fit in were probably polished.
