The Roles and Weapons with the Squad

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That looks interesting and promising. But we are not there yet. So that will not help today's grunts.

Also, I should think that this totally new ammo should be (yet another) opportunity to come up with a better, half way between calibre....

I assume you mean something like the 6.5 grendel or 6.8 SPC? While that would increase the terminal effect (6.8), or both terminal and external ballistics (6.5) of IWs, it would also decrease the "punch" so-to-speak, of GPMGs and LMGs (if the Mk 48 sticks). I feel as if the strength of the GPMG/LMG is more important than the strength of the IW as far as modern war is concerned. Also, the 6.5 proponents who say the 6.5 grendel has superior external ballistics than .308 are comparing open tip/high-bc ammo with a pretty standard ball for .308.

Even when looking for a mid-way point, I don't think the designs shown with the 6.5 and 6.8 are the way to go. They're (relatively) inefficient at increasing energy. A better investment would be developing sabot ammunition for small-arms. A 7.62x39 round generates about 2000 Joules of energy, and has a poor trajectory compared to 5.56. A 7.62x51 with a 5.56 bullet wrapped in a sabot will generate 2400 - 3000 Joules, have a far superior trajectory, far better armor penetration, and it will weigh roughly the same as M43 7.62x39. Essentially what I am advocating is that you should increase powder weight when you want to save weight, not bullet weight.