I visited this topic a few years back and learned that Buffalo Bore Ammo sets their .38 Spl. +P pressure ceiling higher than SAMMI members do.

Current SAMMI: .38Spl +P 20,000 psi

Buffalo Bore: .38Spl +P 22,000 psi

See this link for a more in depth discussion:

Will the real .38+P please stand up? - Page 2 - Shooters Forum