....there are the Bell tests, reported in DGJ. Yes, he used extreme overloads...
....And Craig . . . no, I didn't stick 2 3/4" shells in a gun with 2 1/2" chambers. My first introduction to a firearm was my dad's Eastern Arms .410 single shot, which I still have. (Front bead gone, forend taped to the barrel.) It started life as a 2 1/2" gun. When the 3" shells appeared, what was the solution? Some gunsmith lengthened the chamber to 3"...
Remember, my comment was just my opinion. I believe if you substitute the updated information, my thought remains the same. Isn't it a far stretch to mandate proof houses because your favorite smithing school doesn't like lengthened chambers or to protect the public from over pressure testing.
I'm still curious, what liability have UK proof houses assumed for damage or injury after passing proof.