I read.the change is about penetrating body armour. Evidently the 6.8 does a better job.
From external dimensions, the cartidge is basically a 308-sized case necked down to 6.8 (277). But, it is a hybrid case (stainless and brass). The hybrid case handles pressures of 80,000 psi. This is much higher pressure than either 5.56 or 308. They are claiming that this cartridge outperforms the 6.5 creedmoor, which already far outperforms the 308.
If the cartridge performs like claimed, this will be a badass cartridge that will outpace any current standard cartridge below 300WM.
There has been a range mismatch between US and Soviet weapons that led to, especially in Afghanistan, US soldiers not being able to engage the enemy. It is my understanding that this was the case in operation Red Wings where the Seals didn’t have anything that could match the range of the PKMs, and so they were just fish in a barrel. This new cartridge changes that math, and US soldiers will always have the range advantage.