How could or should the 6.5 mm Grendel be improved?

[stanc]

a. Or is it better to have a caliber that only moderately improves effective range in one theater in the hands of the few who will actually be able to employ it (10% of the Army/Marines)...

How do you know that it would only moderately improve effective range? I've yet to learn of any testing that demonstrates to what degree hit probability would change with either 6.5 Grendel or the notional 6.5 GPC.

...while reducing combat endurance for them across the board in all theaters, and saddling all the soft skills with the same issues of increased recoil, reduced ammunition capacity per weight?

Is it likely to make any noticeable difference on combat performance of the soft skills? Do they currently make a significant percentage of a unit's kills?

Would the reduced machine gun ammo weight and increased mobility for gunners balance out the combat endurance?

You can probably answer that better than I. What is the current ratio of enemy casualties inflicted by machine gunners versus riflemen?

c. With the 7.62 NATO system, SMG's and older era carbines were predominant in the hands of key leaders and non riflemen personnel, not only in the US, but in the European nations and their former colonies. The Bundeswehr with the Uzi and MP5, the Italians with the Beretta SMG, the Sten with the UK, the M1 Carbine with the US (Army Special Forces held onto the M1 and M2 Carbines up until they received the AR15, and the 82nd + Airmobile units couldn't ditch the M14 fast enough.)

I can't speak to the Europeans, but I seriously question that US key leaders and RTOs carried SMGs or M1/M2 carbines in the late-1950s/early-1960s. Special Forces, yes, but not line infantry.

On top of that, the M14/M60 universal caliber system was a terrible failure operationally in Vietnam, due to limited combat endurance and a drag on the logistics system to re-supply units constantly after very limited contact.

I won't dispute that, but remember, those 7.62x51 weapons and ammo were significantly heavier than 6.5 Grendel and 6.5 GPC, which would therefore be less limiting in those respects.

In that light, we might even consider getting rid of 5.56 NATO and 7.62 NATO and do a PDW based off the 6x.221 Fireball for riflemen (KAC 6x35 PDW), the high-powered Grendel for DM's and LMG's, and some .338 Long-Range weapons to replace the .50 BMG. You can carry a ton of rounds with the 6x35, since the cartridges are shorter, while not losing a lot of effective range for practical engagement distances.

Imagine the endurance of a unit that has riflemen/assaulters with 300rd basic loads, with Multi-Role LMG gunners able to carry more than a 7.62 GPMG in less space/weight, with DM's and Snipers carrying lighter, more wind-bucking, less-recoiling blasters, with the Grendel and .338 LM systems, ditching 7.62 NATO, .300 Win Mag, and .50 BMG.

An intriguing idea, and worth investigating. Needs testing, though.

When the discussion encompasses all the systems we really carry, anyone realizes straight away that any illusions of a universal caliber are quite narrow in scope.

Obviously not everyone realizes that, or discussions like this wouldn't be taking place.

The thing is, no army in the post-WWII era has tried the envisioned GPC concept, so there's virtually no evidence to judge if it is a good or bad idea.