You didn'say which .257 you are shooting in the 100 gr XBT, so I'll cover both. In the Roberts the recommended powders are H414 (42.5-46.5)and IMR4350 (42.0-46.0). In the Ackley RL19(46.5-50.5) and H4350 (44.5-48.5). I would try these first before going to the IMR4831.