The original 55gr 5.56 is very meh. It came out of the "more rounds sent downrange is better" school of thought in the 1960s.
The 62gr green tip is perfectly acceptable as a man stopper and does a decent job against armored troops. That's the current .mil issue for the M4 platform.
Some of the contractors went to 77gr for a bit more terminal performance. It's an interesting compromise and was chosen because 62gr doesn't penetrate vehicles as well.
6.8SPC has been used by the Army for the Marksmanship Unit. It's just a barrel swap in an existing M4. It's almost as fast as 5.56, it hits harder and we have tons of performance data on it out of the M4.
That would have been a no-brainer upgrade, but that's not how the logistics pipeline thinks.
@WilhelmIII@heytomjones I know they are not too impressed with the m4 - it does well for everything except killing people.
I think this is a perception problem since the various wars in the ME, due to coming up against the Kalashnikov's, and so they wanted to change it. I thought the plan was a 6.5, the XM7 was simply not meeting standards.