I'll be honest with ya. My 308 Win GAP rifle is a better shooter than I am. It is a true 1/2moa or better rifle. I have shot tons of groups using 3 powders exclusively. Varget, R-15, and 4064. All 3 shoot so well I still can't tell which is best. All 3 git 'er done.
I can say for certain though:
4064 is more temp sensitive than the other 2, and Varget the least.
Varget burns the cleanest.
R-15 produces the highest velocities.......