This prompted the development of both the 6.8mm SPC that the Special Forces have adopted and the 6.5mm Grendel, which, if I remember correctly is ballistically superior to a .308 from muzzle to about 400 yards.
Superior how?
Velocity? Nope
KE? Nope
BC? Nope (loaded with lapua scenars and tested against 7.62NATO then the answer is yes). You can outdo milspec ammunition with a .243 if you load it up with expensive VLD bullets but I don't see the military spending 40-50 cents just for the bullet portion of a cartridge.
If I had to use an AR-15 (not AR-10) to hutn a Moose with, I'd get a .50 Beowolf upper.