I finnished building a load for my 257 on the weekend. I stopped at 71gr of 7828ssc (no signs of over preasure yet) and a 90gr gmx.my average velocity was 3610fps for 5 shots and just a shade over 1" at 150 yards. What was interesting to me was I started at 65grs of powder and wasnt getting a very good burn of all my powder. As I increased my charge it would get better and my velocitys where much closer together. Im fairly new to reloading lots of learning to do yet
In my experience most weatherby magnum cartridges like to be loaded close too or at max to achieve the best accuracy