You may want to rethink maxing out 777 (120 grains) with powerbelts. They like lower velocities usually for accuracy and terminal performance. Anything over 80 grains of 777 is plenty for any muley walking the earth. Try it and see what hits the most accurately. And remember, 120 grains of 777 kicks, a LOT!

Also, I have gotten good accuracy out of the new platinum powerbelts. You may want to give the 270 and 338 grain bullets a try.

Good luck