The proof mark on Krag barrels supposedly showed that after production testing was done successfully with a load rated at 70,000 psi. PSI and CUP ratings are somewhere around 7% of each other, + or -.

So, the "safe" rating of 40,000 CUP does not mean that things start blowing a part at 40,001 CUP. There's some room to spare, Mr. Petrov's experiments not to be ignored, though.