Nice little round the 6.8 - at least mine performs without any issues
The new US issue design looks to be based around the 6.8 bullet, but a heavier on than is standard in the 6.8SPC models.
It's polymer case is very interesting - almost zero heat transfer from the cartridge on firing - so much so that I saw a clip of someone firing an extended burst on full auto, then rest his hand on the barrel chamber...it was cold. Another advantage of the zero heat loss is that less energy is lost, so they are getting the same velocity with a lower powder charge.
The best option would have bene the small-arms series that replaced the No.4 Lee Enfield, until Churchill decided it was not to be & withdrew it, the Rifle No.9 in .280"
It was also given short shrift by the septics at Aberdeen Proving Ground, as they needed to maintain a .30-cal rifle design, for reasons; a design that lasted less than a decade before the 5.56 came into issue = and THAT is another story, if you've seen the recently released Eugene Stoner interviews on YouTube, as the Aberdeen Proving Ground were messing him around to a huge degree as well.