I've got OTT building me a 23" Encore rifle barrel chambered in .256 Maximum (.25-357 Maximum 40 degree shoulder).

Its compares to the .25 Copperhead.

I'd like to shot 90 gr, Sierra JHBT for whitetails in the 100 - 150 yard range.

Anyone have any powder recomendations ?

I have a 12" Bullberry in 256 Maximum. I've been really happy with RL-7 coupled 85 gr and 100 grain bullets

John,
What kind of velocities are you getting ?

I'm hoping for 2,550 - 2,600 fps for the 90 grain bullet out of the rifle length barrel.
Thanks,

Well my Pact Pro decided to quit working and I'm still waiting for it to return from its trip to Pact for the FREE repairs.

So in answer to your question, I really don't know. I've worked up to 17 grains of RL-7 with a 100 grain Hornady Spire point, just to give you a reference point to develop your loads from.