Some years ago lengthening forcing cone was a fad in the UK .
The poof house did some tests and came to the conclusion that it made no or no discernible difference to the guns recoil .
The shooting pundits all claimed it improved a guns handling . As gunsmith I did not care one way or the other but was happy to take peoples money off them .
I took up the question of proof with the London proof house as I believed it should render the gun out of proof . They were very non committal at the time giving answers such as "How would you know if it had been done?" . They did not give any definitive answer so as far as I was concerned they did not say that it did so I took it that it did not .
I do not know what current CIP thinking or rules are on this .