Murphy you ask the following but your private box is full. So here is the only way I can reach you to answer your question.
If you don't mind my asking, what is your best load with a 100 grain bullet for the 257-300. Have you used RL-25 and what do you think of it for the big case/small bore. I have a 6.5-300 to load for and will try to extrapolate from your 257 data. Many thanks,
RL-25 was not as accurate as Retumbo and my best load was 86.grs of Retumbo and accuracy was outstanding at a velocity of 3850fps on the average and my group size was .234". Both RL-25 and Retumbo produced about the same velocities. Both could give 3975-4000fps but accuracy was better with my barrel between 3800-3900fps. I hope this helps.