Ted,
Here's a scan of page 137 where I got my information from. Yes it refers to black powder, but peak pressure is peak pressure no matter whether black or smokeless powder was used.
FWIW this same document on page 136 also gave the information you quoted as well.
Steve
Also on my 12 gauge I like the heavier proofs because I feel they allow me to safely shoot heavier loads occasionally, if that's all that is available (such as when I was hunting ruffed grouse in the Alaskan boonies and ran out of my light loads).
