Not to sound preachy but as long as you're limited by the size of the M16 magazine, you're stuck with the limits of physics. This is an engineering issue, thus mathematics. The time to have made a cross the boards switch was before everybody bought buttloads of new rifles, say 2002-2004 or so. USMC was just buying M16A4, Army was still switching to M4's IIRC, SCAR wasn't even started yet, so a switch could have been total. Now with all the new hardware and decent performing 5.56ammo (Mk262, Mk291, Mk318, etc), there really isn't much demand left. Having service specific rifle ammunition went away with the straight pull 6mm Lee rifle and there's no reason to bring it back AFAIK.
At this point, switching the US DOD over to another rifle caliber would simply be a MIC welfare program. Maybe in 10-15 years when we move away from what we've currently got, but by that point maybe the Case Telescoping Ammunition (CTA) thing will be sorted out. At least the Euros are currently paying the way for most of the fielding of that technology. 40mm CTA cannon for their new APC's. Might be the only defense research weight they ever carry
The NRA guys shooting spaceguns who are not using 5.56 seem to like a 6mm bullet with the 6.8SPC casing for increased powder capacity but they single load for the prone stuff and so are not limited by OAL. A Grendel casing would be the same thing only slightly moreso, as it has more volume IIRC. 6mm bullets are significantly better ballistically than .224 bullets and you don't give up huge amounts of MV as with high BC 6.5mm bullets. S/F....Ken M