57 grains should get around 2500.
We can usually predict what velocity a minor change in powder charge will bring by dividing the "new" charge by the old charge, then multiplying the original velocity with the result. Thus 57/59 = .996 x 2590 ("almost" 2600) = 2502.
This formula works very closely with single-based powders, not quite as well with double-based. But it is generally pretty close with minor charge variations.
One thing that may be going on is that all powders do vary somewhat from lot to lot. I went down to Charlie Sisk's shop in Texas a few years ago and we ran a bunch of pressure/velocity tests. One of the rifles we used was my CZ 9.3x62, and we tested some of the loads I normally use. At that time one of these was 62 grains of RL-15 and the 250-grain Nosler Ballistic Tip (now the 250 AccuBond). Everything was identical in the load we tested, except Charlie's lot of RL-15. It took two more grain of powder to equal the velocity I got with my batch of RL-15.
This sort of lot-to-lot variation isn't uncommon. Add that to using a different bullet that might very well create different pressures, and it's no wonder that sometimes data can vary 5 grains or more between different sources of data.
We did confirm, though, that velocity is the best predictor of pressure for the average handloader.
Did you use the same chrono, or did you use Mr. Sisk's? I am assuming (I know, I know) that there is a difference, minor though it may be, in different clocks.
