you will probably be hard pressed to find a load that will shoot with the factroy stuff as they use differant powder than whats available to the public.
I reload for a buddies 25-300 Win mag which is just a step up from the 257 wby , I've found that R-22 and VihtaVuori N-165 tobe very good powders with the heavier 115 and up bullets. Also the N-560 will give to better velocity especialy if you have a longer barrel
I'd start at 65grs with the R-22 and 70 with the N-165 and work your way up , and BB has a point about the smaller primers , your rifle may like a little cool start
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