There is no proofhouse "minimum" for wall thickness. The British gun trade recommends a minimum, but a lot of guns have passed proof with MBWT less than .020.

As the above graph illustrates, peak pressure always occurs within the chamber. That's true whether we're talking black or smokeless powder. And it always declines quite rapidly afterwards. Sherman Bell's more recent experiments with a variety of test loads showed basically the same type graphs as the Dupont tests from, I believe, the 30's.