There is much contention about which round would/should replace 5.56 NATO. I doubt that anything will at this time due to our huge investment and general funding climate. If anyone does first I think that it will be MARSOC.
Anyway, if you follow the 6.8 debates on the internet you often hear that the 6.8 is a military-used or tested cartridge. Detractors usually say that it is not true and that there is no evidence that it was ever used despite that some of the folks stating first-hand knowledge are pretty credible. (Not to mention the comments from companies like Black Hills and Silver State Armory). But, as written proof I submit the following quote (and link and pdf) from folks that know a little something about M16/M4/AR15's:
LMT Press Release
The 6.8 has been used by Special Forces troops in Afghanistan with much success. As the obvious benefits of the 6.8 become more apparent, the US Army is taking the caliber under consideration to replace the 5.56 currently carried by US troops. The 6.8 has proven to be more accurate at longer ranges and have greater hitting power than the 5.56.
LMT should be known to anyone posting anything on an internet forum discussing the AR platform. If not, a quick Google should fix that. They are pretty well regarded :)
Yes, it is a little old (2006) but I am not arguing that it is going to be replacing the 5.56 just that it had been used/tested.
Download the PR (pdf) : PR_LMT_6_8_barrel
