The Alliant website boasts RL15 was selected for M118 LR
. Has something changed? I just figured that since ATK is the current LC contract manager and they also own Alliant, that they'd probably pushed to get that powder accepted.
I once pulled a box of the old GMM 168 load, and it had 43.5 grains of IMR 4064 under the 168 grain SMK. The claimed velocity was 2650 fps from a 24" velocity test barrel, same as it is for the RL15 version today (ATK also owns Federal now). QuickLOAD predicts 2648 fps, which is so close that the difference is utterly insignificant. So I know the IMR 4064 used in the rounds I pulled was very close to what's in the QuickLOAD powder model. That good agreement means that even if Federal was using a bulk grade IMR 4064 with a wider burn rate standard deviation, that the particular lot I weighed happened to fall right into the same burn rate range sold to reloaders as canister grade IMR 4064. The charge weights were ±0.2 grains, for a span of 0.4 grains variance, which is very good for a volumetric throw of IMR 4064's long grains. The loads were very slightly compressed. They were in Federal .308 Match cases with Federal 210M primers, obviously.
If I wanted to duplicate that load I would start by firing a whole box of purchased Federal ammunition over the chronograph. Then, starting at 41 grains, in no greater than half grain steps, I would work up toward the same velocity average using the same brass to see what the charge weight was with my lot of powder. Then I would switch to tougher brass for longer reloading life. Remington for every day shooting and Lapua for match critical loads. With both I'd be starting back at 41 grains of charge weight and working up just as before, checking to be sure the different cases didn't show any pressure signs reaching the same velocity, either.
I would also be double-checking to be sure accuracy stayed as good as with the commercial ammo. Sometimes when you roll your own you get worse before you get better. Search the sight for load work up and seating depth workup and other factors.