Great choice of cartridge. I've owned and loaded for a gaggle of 257's. I much prefer the heavier bullets as they buck the wind and don't run out of steam as fast as the lighter bullets. My go to powder is RL-25 with a 115 NBT. It has consistently provided the best combination of accuracy and speed across the range of 257's I've loaded for. Additionally, I've found the 115 NBT to hold up well at 257 Wby velocities. My second choice would be H-1000 coupled with a 110 AB. Also consistently accurate and fast. As an aside though, I have found the 110 AB to quickly pressure spike when at or very close to max. Nothing terribly significant though. Just approach the upper end with caution.
