Chuck, it's correct that lengthening the cone doesn't take a British gun out of proof. But lengthening the chamber does. That's where the peak pressure always occurs.
Going back to my post about Flues frame failures, lengthening chambers would only increase the likelihood of those if, as a result of the longer chambers, someone were to use higher pressure shells. But most gunsmiths that specialize in doubleguns, I think, would like to see a minimum barrel wall thickness of .100 at the mouth of the chamber. I don't know whether another 1/4" added to the length of a 20ga Flues chamber would put you below that mark, but I think it would be something to consider.
This is great stuff to know! I was not aware of the proof distinctions between chamber lenngthening and forcing cone lenghtening.
