My estimation for the 6.8/M4 is that is would not significantly add PB range, compared to the M855/M4 (again, same platform, different caliber).
Would be interseting to see actual test data.
Remember that the Soviets, who'd been fielding the AK47 and AKM with a 16-inch barrel since the early 1950s, have also had unhappy users insofar as the smaller bullet of their own downsized 5.45mm AK74- including MikTim Kalishnikov hisself.
What I'd really like to see would be that bullet from the 6.8 SPC utilized in the 5.45mm case, thereby suitable for either the 5.56x45mm-length actions, or the former SovBloc &.62 M43 and 5.45 weapons.... and maybe even the existing magazines. I haven't run the ballistics of the reworked round yet, though I'm sure the Soviets did when they worked out the 5.45 cartridge. But I have offered the idea to someone in that line who's interested.
Archy;
There are are 6X223, 6.5X223 and 7X223 wildcats which generally are poor performers with bullets of any appreciable increase in mass/BC.
The 6mmX223 can push a 70 grn 6mm bullet at about 2700fs, much less effeicient than the 77/80/90 grn SMK already in use in the 223/556, don’t even try seating a 107 SMK with the expectation velcoity past 2000fs!
Again, no free lunches-trade capacity for bullet diameter.
You kinda lost me-use the 5.45x39 mmm with a .277 diameter bullet? You would be losing more case capacity than if you wnet with a 6.8x223 (45mm case), I think.
There are a few dedicated 6mm cartridges being used in High Power Competition in the AR-the 2007 National Match Rifle Champ (Carl Bernosky) used a 6mm Hagar, not too sure of the case length, but the OAL must be AR mag length for use in the rapid fire stages....
Bottom line is the 233/5.56 mmx 45 is probably the most mature cartridge since the 308/7.62 (full course 200-600 yd match competition drives designs to the edge of performance, occassionally past it!)
Nothing to stop you from creating a 6.8 x39 mm, but it will be a lessor cartridge than the 6.8 or the 5.56 already are;
Rough interpolation would indicate that it could not exceed ~2450-2500Fs (gas law rule of reduction of bore diameter compared to existing rounds always yeilds less velocity from a similar platform profile) with a 115 grn bullet, and AK/SKS platform accuracy is nothing to boast about either.
The 5.56 with advanced bullet design (77 SMK) still is the best combination of performance, capacity, reliability and accuracy compared to the other combat options.
If you want a 30 cal (762x51, 762x63) performance step up to a proper platfrom, remember.... no free lunches for the dogs of war! (Democrat party planks notwithstanding....)
Molom Labe!
Feed reliability partly comes from the fact that bullet diameter is less than case diameter.
But, I was thinking earlier that maybe a good idea would be to neck up a 223 to maybe a 25 caliber, then load a sabboted 22 caliber bullet in it. Also, very fast burning powder, polygonal rifling, increased head space, less twist, much higher chamber pressures, aluminum cases or steel if necessary and a hammerforged stainless barrel. Heck, make it out of monel or nikasil or whatever stands up better than stainless. Invent a new metal if necessary. Whatever it takes to increase pressure and velocity.
But not at the expense of accuracy. Some of the ideas I just threw out may hurt accuracy.