I've found my most accurate load in my .257 is the Sierra ProHunter 100 gr on top of 45 gr of IMR 4350. OAL 2.865"
I use a Remington 700 Classic. I also get MOA with the Sierra 87 grain Spitzer on top of 41gr of 4064.
The .257 Roberts really shines when handloaded !! Hope this helps. G Lynch