From everything I've read here (and elsewhere) in numerous discussions of proof, I would have said no. However, someone recently brought the following quote to my attention, from McIntosh and Trevallion's "Shotgun Technicana", p. 78:
"English proof law holds that deepening a chamber renders a gun out of proof, but simply lengthening a forcing cone does not. (Or at least this is the London Proofmaster's position; we're told the Birmingham Proof House does not permit forcing cone alterations.)"
Based on the above, a friend who recently had forcing cones lengthened on a vintage British double emailed the Birmingham Proof House. He received the following as part of a brief reply from the assistant proof master:
"The gun would be out of proof by removing material from the forcing cone."
Mr. Trevallion, Dig, Hugh, others familiar with British proof practices . . . have we been wrong all along when stating that lengthening cones does not invalidate proof? And since there is no standard length for forcing cones (unlike chambers), and since there is no proofmark specifically relative to the forcing cone, how would the proofhouse know for certain that the cone had been modified?
