It has always been my understanding that lengthening a chamber cone SHOULD render a gun out of proof as there is metal removed from the barrel . But came the argument ,providing the actual chamber has not been lenghtend byond the proof limits how dose any one know it has been done ?So it was not a consideration and as no directive was issued no one bothered .But Proof law is fickle, CIP has changed thinking in some areas. for instance I have threaded dozens of .22LR barrels to take moderators with out any problem ,Then a few years back it was decided that "as this could subsancialy weaken a barrel" these guns are now out of proof . This has been argued but to my knowlage no test case has been defined .
With proof we are at the discretion and mercy of the proof masters to some degree and often new proof masters make new policy . The answer to the original question must be yes it dose as a technical point but a point that has never been enforced .
