The military looking into a new 6.8 isn't a rumor. A large grant was issued to 5 manufacturing concerns to develop weapons systems and submit samples for testing/evaluation.
The 5 are FN, ATK, H&K, PCP, and a fifth I'm not familiar with and can't remember.
For the personal weapon, it'll be similar to current M4. The real question is going to be the SAW (squad automatic weapon). It's performance and reliability will determine if the program moves foward.
My source suggests that it will be either the .270AR or even based on the .30REM AR case. Either way, Performance is a step above the 6.8SPC.
Perhaps it's even the long, eventual, final development of the telescoping case.
But, it will be firing a monolithic bullet like the M855mkIII (like a Barnes TTSX, except tip is a hardened steel penetrator, instead of plastic).
Long range performance is said to be equal to a 6.5, only with better wounding charactistics on soft targets.
It's going to be interesting to see what it is.
Lots of tight lips and rumors running around Ft Benning and the H&K folks.