There are a number of NATO standards, but the thing is that nobody is really happy with the 5.56 SS109 and successors any more and it looks like the 5.56 is too small to do what people are wanting it to do now. Many 5.56 weapons may be convertible to 6.8, though.
Fact is later/latest 556 rounds like the Mk318, the Mk 262, the M855A1 and a few others are vast improvements over the SS109 bullet/M855 round.
The biggest problem I see with all this 6.8 interest is that the platform ( the AR15 base) is not really adequate to eat a biger round that actually produces significant increases in both trajectory and terminal energy.
As it stand, the 6.8 from a 14.5 or even 20 inch barrel does not show any improvement in flatness of trajectory, equating to less hit probability with improving the marksman generally. Range is important. The current 6.8 SPC (115 grn FMJ)can do better than the 556 M855A1 (65 grn hybrid ) at the muzzle, but it loses at 500m, both energy trajectory and penetration. Actually, the M855A1 beats the 762x51 M80 147 grn ball, however, that too has been upgraded ( M80EPR).
This new effort hopes to mate hybrid cased rounds ( steel base, brass or maybe even polymer body, improved design bullet like the M855A!/M80 EPR) and improved propellants.
The telescopic or caseless - no, I doubt we will be going there in this effort- since the 60s, this has been a dead end.
Currently, the 556 nato round runs at 55k psi ( EvAP or SAAMI method) 63.5k psi CIP/NATO methods ( actually, both the same, just measured at different locations/methods)), but the M855A1 is loaded to approximately 68k PSI, exceeding both test spec methods for max pressures. The short barreled M4s loose performance due to the short barrel ( 14.5inches) but certainly exceeds the M855 levels. The result of this higher pressure is increased wear of both barrel and bolt/carrier group.
Propellant technology and metallurgy has not advanced enough to realize much improvement at present. Until then, the 6.8 effort is an R&D dollar dump, as we have seen with about 6 other small arms efforts in the last 20 year.
The AR15 platform is too small to see a cartridge that improves significantly on current weapon/ammo, the AR10 too big for what is asked, maybe the “”, shooting an intermediate round (okay, if has to be 6.8/277, fine), but perhaps around a 47mm case, with a 6.8 case head ( 25 rounds in a 556 30 round mag instead of 20 round in an M110/AR10 mag) that pushed a better bullet than the 115grn 6.8 ( I like the 6.5 130 grain family at about 2900 fps from a 18 inch bbl personally).
Of course, all this would require a new platform, and that would cost billions. Whilst the AR15 base is already on hand....
We shall see.
Just replace the barrel, bolt, and magazine and your ready to go.
This round is not 6.8 SPC.
You can already purchase 6.8SPC II ammo, barrels, bolts and magazines to refit an AR-15 pattern rifle. There are specialty rounds for hunting.
I have a .224 Valkyrie that uses a 6.8SPC II bolt and magazine since that is the parent case.
The military may take a different direction, but the existing builds prove that it is feasible. I would include headspace gauges for go/no-go/field if converting an existing rifle with parts from a supplier to ensure a good mating of the new barrel and bolt.