Frankly, there is no reason why a second proof load should destruct a barrel when the first one did not even cause a Plastic Deformation. Now perhaps they do not test for plastic deformation until AFTER the second firing. On the other hand, if the first firing caused a plastic deformation, the second firing could, indeed, cause a catastrophic destruction. If you get a gun back after a plastic deformation and before destruction, you can always sleeve the barrel and resubmit. A new gun should not fail. Perhaps a FULL-TIME Proof Master should be hired. If I were a London gun manufacturer, I would be severely PISSED.