If we get a cartridge that weighs less than 7.62 M80 ball and may do everything it can do but slightly better is something worth considering.
Consider? certainly. But also consider the cost vs the benefits.
And some of that cost cannot be yet quantified or calculated. Because the cost of the weapons system includes more than just the cost of the rifles and ammo. Training, manuals, cleaning kits, spare parts (and which ones to focus on ?) and training not just the users but also all the support troops involved, both supply and repair.
All these things need to be figured into the costs, and many of them cannot be determined until after the rifle and round are in general service use.
I have faith the 6.8mm will give better down range results than the 5.56, but I am not so convinced about its superiority to the 7.62 or if the degree of improvement is worth the cost.
Consider this, the 6.8 might be better at penetrating body armor than M80 ball, but is it better than 7.62 AP??
Look at what we did in WWII for example. By later in the war, nearly every BAR gun was fed AP more than anything else, and in some places it was AP only.
If the concern is penetrating as yet unknown body armor in an as yet unknown future conflict, why wouldn't switching to AP be a reasonable (and much cheaper) solution??
Isn't that what the M855 5.56 with its penetrator insert designed to do? Defeat the expected body armor of Warsaw Pact troops in the anticipated WW III?
So, at the moment, we have bought/are buying a rifle and round based on what we are being promised it will do. Once we have the rifles and ammo in hand, and in use, THEN it will be time for further discussions.