OOE,
The 6.8 SPC is proofed and ready for combat. From what I understand at least one SFG took 6.8 uppers down(on the 2 way)range. Open source stuff indicates that the improvement at the +250 meter range didn't justify the loss of magazine capacity, and lengthened follow up time between shots. Controlled pairs came slower. I believe the USMC gave some thought to adopting the SPC, but economy of scale issues put the kaibash on it, maybe one of the former Jarheads around these here parts knows more about it.
All in all, a good,
relatively more powerful cartridge firing a heavier (115 gr. ball) bullet, with
slightly improved barrier penetration and
marginally improved ballistics at the +250 meter range (out of a 14.5" bbl.) than Green Tip. But with the negatives stated above. I would posit that the way forward is not the adoption of a new caliber, with the logistical nightmare across the board that would entail, but the widespread adoption of a heavier 5.56 round such as the
Mk262, already entering in limited service in the SDM role.
Thanks,
Cato