Brittany Guy: just to clarify, British proof law does not require reproof after lengthening forcing cones alone.
This may not apply to light weight small gauge doubles, but certainly with most U.S. 12s, the angle of the forcing cone is more abrupt than the exterior taper of the barrel, so the wall thickness in the cone (in my measuring experience anyway) is usually equal or even greater than the end of the chamber.
An illustration from the Hallowell site



This disaster was from "inexpert" lengthening of both the chamber and cones


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