According to The Marine Times, the head of Joint Forces Command was trying to get his troops a better rifle with more powerful bullets well before the Corps deployed in Afghanistan. Pre-theater, General James Mattis’ lobbied for a 6.8mm round. Yes, well, the Marines recently “settled” for partial deployment of 5.56mm Special Operations Science and Technology (a.k.a. SOST) ammunition, to fit existing weapons. “The Corps is still considering a swap to larger calibers, but if SOST continues to show promise, it may not be necessary, said Chief Warrant Officer-5 Jeffrey Eby, the Corps’ senior gunner. Marine officials ’100 percent trust’ the new round, he said, and are awaiting feedback from operating forces who are beginning to use it.” (Never mind the AA-12, of course. Or the 416.) Question: why are the Marines so reticent about switching to larger, more powerful and accurate ammo? Recoil. More specifically, not-so-strapping Marines. “Fielding 6.8mm ammo also would result in new marksmanship challenges. Much like the 7.62mm M14, a 6.8mm rifle produces larger recoil than an M16A4 or M4, making it difficult for smaller Marines to keep the weapon on target, Eby said.”
I have a 6.8 SPC AR-15, and the difference in recoil between it and my .223 AR-15 is hardly noticeable. Even for a female shooter (probably the implied reason), it shouldn't be a big deal.
I think the holdup is just the commonality/logistics/supply chain issues; also, nobody will mistake a 5.56 gun for a 7.62 gun, but because 6.8 SPC AR differs from a 5.56 one only in the bolt, barrel, and magazines, the chance for confusion is much greater.