What Ted said.
ANY chamber/forcing cone alteration is considered by the British Proof House to take a gun out of proof.
Joe is correct regarding forcing cone lengthening; slightly increase volume within the chamber/cones = slightly decreased pressure. At least Bell's study (WITH 12G - we have NO data for 16 and 20g) suggested such.
I would not lengthen the chambers of a lightweight British game gun, of any gauge, and would do so with caution (and only by an expert) if a U.S. maker's 16 or 20g.