Right now both loads of sniper ammunition are being issued side by side. Mk316 was developed by Crane for SOCOM, and the regular military still uses M118LR with Re15.
Right now the Army has a PIP (Product Improvement Proposal) for M118LR, which the ammo developers at Crane have started shaking their heads, considering that Mk316 is already in the pipeline. It is funny, but the best accuracy loads the military has in the inventory all came out of the Navy, which is arguably a smaller user of small arms than even the Air Force.
Both loads must meet the same accuracy standards, Mk316 was developed to eliminate some of the temperature variability that a shooter/spotter would have to account for to make a first round hit at extended range.
I don't know what powders that Crane tried before settling on IMR4064.
Machine guns are awesome until you have to carry one.