Knowing that Armalite and now DPMS makes the AR in .243 Win, I pose this:
There is much discussion and debate regarding a so called replacement cartridge for the .223. I know the biggies are currently 6.5 Grendel and 6.8 SPC. Frankly I think the .243 Win is far superior to all the above for an infantry type cartridge.
Everybody makes it, ballistics are fantastic. What say you?
I'd say you need to do more research into 6.5mm bullets, which have some of the highest BC available.
ETA - in the AR10T platform the 6.5mmX08 (commonly called the 260 Rem) will smoke your choice at any range, and btw don't spam the board....