I have shot the 257 Wby for many years. I have two both Mk V one with factory barrel and one I rebarreled with and Obermeyer 6R (yes you read it right 6R) both barrels are 26 inch length. I use IMR 7828 since it was introduced in the early eighties and I use 117 grain Sierra Pro-Hunter flat base bullets in front of 70 grains of 7828. Primer is a Federal 215 Mag. Both rifles will shoot this load close to an inch and I have used it to take many head of game. In my experience I cannot get any boat tail bullet to group anywhere near as good as the flat base ones. The first loading data that came out from IMR shows the 70 grain load going 3496 fps but I never got within 70 fps of that figure in either rifle. I suspect that the boat tail bullet lets early powder gas to escape around the base while the bullets travels the .378 inch free bore length. Flat base not so much. This is just my theory based on my experience. I will try IMR 7977 when stocks of my 7828 run out. YMMV
Back in the 90's when I loading for the 257. I had Kleinguenther build me one on a k15 action. The old man being German hated to buy factory ammo to test rifles. Ask me to load some 257 for him. His load involved the Sierra 117 flat base with H4831. As I recall about the middle load in the Speer manual. Shot real well in the two 257's I owned . Never shot this load over a chronograph. Hasbeen
