DRJ,
I doubt that the proof pressure would be above 30% for rifles chambered for higher pressure cartridges.

I think that the "30% to 50%" does not mean that ALL rifled arms are subjected to proofing above 30%.

I think it means that lower pressure cartridges like for example handgun centrefire, might be proofed at the higher end of the 30-50%, and that higher pressure cartridges like .375 H&H etc might be proofed at the lower end.
Once you are dealing with cartridges producing pressures above 60,000psi, a proof load of plus 30% is going to be pushing the brass cartridge case close to or beyond its limits.

I have also read that the CIP plus 30% proof, was lowered to 25% in 2006.

Out of interest, how old is the "Standard Directory of Proof Marks" book? What is the year of publication?
I don't have that particular book, and am wondering if it might not be up to date with the current proofing regulations.
Cheers.