No way.

Ain't gonna happen.

The 6.8 is a nice, light recoil hunting cartridge {as is the 7.62x39}, but the vast majority of After Action reports and Lessons Learned indicate the 5.56 is just fine. There are, I might add, a wider variety of loads available in service now and more or less everything from 55 grain M193 to 62 grain M855 to the 77 grain HP has been fielded. Some Spec Ops units will play around with whatever they can get {6.8, non-standard 5.56 loads, 7.62 Nato, etc}, but overall Uncle Sam is pretty happy with the 5.56.

It should be mentioned that the compadres of the flea-infested ne'er do'wells that are fertilizing ever-greater tracts of Mideast deserts and Central Asian hillsides do not like it one bit.