L.Brown,
Lengthening the forcing cone results in a more gradual angle of transition between the chamber diameter and the bore.A more gradual transition between chamber and barrel diameters will result in a reduction in stress concentration in the barrel wall.In my opinion, lengthening the forcing cone will almost certainley increase the ultimate strength of a sound, in proof barrel.
One would think the Birmingham proof house would want to confirm the impact of lengthening forcing cones on barrel strength before making any ruling on the issue.
