Couple comments on forcing cones and proof:

I can't find the reference now, but I recall reading somewhere that the Birmingham and London Proof Houses have different views on lengthening cones and proof status. I seem to recall that Birmingham does not consider a gun with lengthened cones out of proof but London does, but I may have that reversed. I recall that the information came from a good source but cannot find it at present.

While the chamber itself is the area of highest pressure, Sherman Bell's experiments established that simply lengthening the cone (without lengthening the chamber) reduced some of the increase in pressure that results from firing a shell that's longer than the chamber in question.