In this country, "proof" is accomplished with a wall thickness gauge.
This. Why risk blowing up a fine old gun if you can just measure wall thickness instead. If they go .025 or more, you're good. Less, and it's wall-hanger. Easy.
JR
I regularly shoot guns well below 25 thou that are in proof - and safe with the loads for which they were designed and proved. Lots of fine guns had quite thin walls when new, especially pre WWII when light guns were popular.
Good grade steel (and Damascus) can be well below 25 thou towards the muzzle with no risk of burst, but they are easily dented.
