Keep in mind that even if Federal uses RL15 for their GMM, or if Lake City uses it for M118LR that it is not the same RL15 you get at your local retailer.
They use non-canister grade powders that can vary from lot to lot, and adjust the recipe to get the specified performance.
Canister grade powders that you and I can buy are remarkably consistent, and a can you get today will most likely be close enough to one you bought a couple years ago that you couldn't tell them apart.
If you really want to duplicate the load, shoot a box across a chrono and get the average velocity. Then load Gold Medal brass, with Gold Medal Match LR primers, and a Sierra Matchking and tweak your load to get the same average velocity.
Honestly though, unless there is a really specific reason to duplicate it, you are almost certainly better off working up a load on your own.
For .308 sized cases, I have always had excellent luck with IMR4064.