The general opinion is that barrels should have at least .025" MWT but this has nothing to do with "proof". Many English guns with less than .020" MWT pass proof both with fluid steel and damascus barrels.
A great number of English guns did not even have .030" MWT when they first left the factory but all had to pass proof.
I have a Greener with Siemens fluid steel barrels with .020" MWT and they passed re-proof. Would they stand a dent removal? Probably not, which is why the recommended .025" MWT for better durability and longevity.