Just a question ... since stopping power is in part a function of energy delivered to the target, is it possible to get better results by just creating a heavier 5.56 round?
The cartridge has reached it's limit (especially in shorter barrels) with the 77 grain ammunition issued to the SDM's. Anything heavier can't be accelerated quickly enough through a short tube.
The projectiles will also be at the limits of spin-stabilization out of a 1:7 barrel. As a heavier bullet gets longer, more spin is required to keep it stable. It's been a while since I did the math but I believe the heavy bullets are turning something a bit over 300,000 rpm.
There are quite a few good bullet / cartridge combinations in the 6.5 - 6.8 mm range that could be made to work on the same platform. However, I doubt that the average soldier could make use of the extra horsepower.
Snipers who use an M16/AR15 platform use 77gr Sierra HPBT's (this is 15gr heavier than M855 (62gr), a 24% increase in weight and a thinner jacketted "match" bullet).