I entered the formula from MBC website, and suggested hardness @ 50k psi comes out to a whopping BHN 39.I'm sure some of my loads have been up in that 50,000 psi range.
I use non GC'd 18 BHN for full tilt 357 loads with only minor fouling, myself. So I'm not sure that the formula is all that complete. Hodgdon's jacketed data for the charge suggests 40k psi. Cast seems to affect psi in unpredictable ways, though. Sometimes it goes up; sometimes down.
Just one of many factors is cast bullets are often slightly longer due to the lube grooves, reducing case capacity with the effect of increasing pressure. But that would be more of an effect in a small case. In a roomy 357 case, maybe I'm not getting the full 40k psi. I can't find my Lyman manual to check, so I'm not sure if any of my cast rifle loads even reach 50k psi!