There is no way to know what powder that Winchester uses in their load. Assume that it is something that you can't get commercially. Possibly something similar to Winchester 780 Supreme powder.
However, there are some excellent powders available for the .270wsm.
I'd start with the new Alliant Reloader 17. It alledgely is developed and intended for the new short magnum cartridges.
I've gotten superlative results with it in the 7mm08 and .260Rem.
Hodgdon H4350 has also given others excellent results in the .270wsm. I'd start with either it or RL17 as these will give you the closest to factory performance. Hodgdon suggests that H1000 will give the highest velocities. Get a copy of their data online, or one of their publications.
You'll need a chronograph to verify that you are indeed getting "factory duplication". Your rifle and factory ammo for baseline data will be essential for determining exactly what "factory duplication" really is in your case.
