If barrels with 20 thou wall thickness were regularly failing the proof they would not mention that as the recommended minimum. The facts are that barrels under 20 thou regularly pass proof and are deemed safe. Barrels with 20 thou and all other characteristicds in good shape pass proof in overwhelmingly high numbers. It is extremely rare for failure in the rigid proof testing for barrels in excellent condition because of wall thickness of 18 thou or above. Barrels fail for other reasons, but not often from bursting or changing bore diameters as in bulges. AND REMEMBER, THIS IS WITH 18,500 PSI LOADS!
Jeweler, those heavy proof loads you mention would only be used if the owner or maker is requesting magnum proof (now called superior, and marked SUP under 2 crowns) on the gun in question. Standard proof (now marked STD with one crown), which is the old 850 bar proof, is about 14,000 psi--and that's the proof to which most vintage British game guns would be subjected.
