There's not a ton written about this that I can find, but cracks are appearing in the US military's plan to replace the M4 with a heavier battle rifle designed to defeat body armor. Sig Sauer, which also holds contracts for the current generation pistol and LMG, won a contract last year to also provide the US military with a new rifle. This rifle is to be called the XM7. The civilian equivalent is the Sig Spear. Calling this thing an assault rifle is a bit of a stretch, as it is very heavy and uses heavy ammunition. It turns out that the rifle is failing to penetrate modern body armor without the use of tungsten bullets. Also its reliability is questionable.
In the end, yeah.
Basically the army wanted to defeat level four body armour, and no 5.56 round that the M4/16 fires, even the armour penetrating ones, can do that. So they want to replace these guns.
The only round that was available that can be fired by assault/battle rifles, that can defeat level four armour is the modern black tip 7.62x51mm round with a tungsten carbide core. The SCAR-H can fire this, it is chambered in 7.62x51. But the army and sig thought they could increase the efficiency of the 7.62x51 to a point that it could defeat level four armour without needing the tungsten core. That is what the point of the 6.8x51 round, and the new Sig rifle was.
It turns out the increase in efficiency is not enough to penetrate level four armour. They still need the tungsten core to do that, even with the 6.8x51 round. So the army is in the exact same situation as before, needing special ammunition to defeat the level four body armour. Only now with a gun 2lbs heavier than the SCAR-H, firing some over-engineered ammunition that's no better than the ammo the SCAR-H fires.