Lengthening a chamber definitely requires reproof. Lengthening just the forcing cone, however--which sometimes is all that is necessary, with older guns that have very short, sharply tapered cones and present problems even with the 67MM shells approved for use in 2 1/2" guns--does not necessarily require reproof. According to McIntosh and Trevallion in "Shotgun Technicana", the London Proofhouse states that just lengthening the cone doesn't take a gun out of proof, but the Birmingham Proofhouse holds that it does.

Dig or any of our other British contributors: any comment on those conflicting views?