statement is over 30% - at least 30% - Kuduae guessed it might be as high as 50%.
Probably ranges that much, case to case due to taper and chamber conditions.
A chamber that is rougher- ie: reamed but not polished, which is normal, will produce less pressure than one that is polished. Then come pits in old guns being re-proofed - I firmly believe there are too many variables in merely oiling cases, even if the same oil is used. Of course, they are using a copper crusher, so get pressure readings each shot when testing the load in the crusher apparatus- must remember that. However, they are then oiling the ctg. case and putting it into a different chamber and fireing it, along with the possibly different conditions, which also includes different dimensions of chamber body, leade and throat diameters and lengths.