Nothing wrong with the 6.5x55, but it is not what the US military demands. First, it has to be short to work through a short action, probably around the same length as the 5.56. Second, it must be somewhat more powerful than the 6.5x55. I haven't seen anything on exactly what bullet and ballistics are required, but there are references to its using a bullet of around 120 grains, probably designed for extreme penetration ability, at a MV of around 3000+ ft/second from a short barrel. I know that one of the candidate rounds has been reported as having an all-plastic case.
I did some of the very first evaluations of the 6.8x43 (6.8 SPC) for the USAF back in the mid-2000s. It was very impressive, and I still have someplace some of the Remington "White Box" ammo made up especially for our tests.
I'm afraid the US military is on the verge of letting the pendulum swing way too far in the other direction.
I agree with you about the velocity being north of 3000 fps. One of the design goals is better armor penetration and what gets that done with a bullet built similarly to the SS109 will be increased velocity well above the current 2970 fps for the M855A1 EPR.
The three contenders all seem to be slightly shorter than the 7.62x51 round and that's consistent with a high velocity 6.8mm 120 gr round, although I think it'll still have a higher than customary pressure to get sufficient velocity in a 13" barrel.
That in turn will mean excessively deafening noise levels that would make a suppressor or XM177 style moderator imperative, adding length and weight.
But…the major mistake about to be made is to return to what is in essence a full power battle rifle round, which will again compromise control of a select fire weapon, as well as increase the basic combat load for the average infantryman.
Post WWII the Brits wanted to adopt the .280 Brit for the FN FAL as the NATO round and they compromised with the .280/30 which shared the same cartridge head as the .30-06 to make it easier to make the new round on existing tooling. When the .280/30 was rejected due to too much drop at 800 yards, they upped the velocity to 2700 - 2800 fps. However, the US still forced the adoption of the 7.62x51 - and then within a couple years of adoption by NATO allies started developing the 5.56x45 round.
5.56x45mm M193 actually wasn't bad in terms of trajectory or lethality at practical ranges and armor penetration beyond 500m could have been further improved by a move back to the original Stoner/Sierra developed "Type B" bullet with its higher ballistic coefficient.
Instead however, the heavier 62 gr SS109 round was adopted to improve long range penetration, which was largely counterproductive as the higher projectile weight lowered velocity, which reduced penetration. Then the US military doubled down by first introducing the over weight and poorly balanced M16A2 and then, after recognizing it was too cumbersome, adopting the 14.7" barrel M4, which further reduced M855 velocity, penetration, and lethality. They gained some of that back with M855A1 EPR round, but not much.
Now the US Army wants to correct those faults by going back to what amounts to a full power battle rifle round. Consider the following:
7.62 NATO M80 ball in an M14 (20" barrel);
Charge Weight: 41.0 gr Muzzle Velocity: 2800.0 ft/s
Firearm Weight: 9.2 lb Bullet Weight: 147.0 gr
Output Data
Recoil Velocity: 9.4 ft/s Recoil Energy: 12.6 ft•lbs
Recoil Impulse: 2.7 lb•s
Versus
New 6.8mm round at an estimated 3200 fps fired in an 8.5 pound rifle;
Charge Weight: 41.0 gr Muzzle Velocity: 3200.0 ft/s
Firearm Weight: 8.5 lb Bullet Weight: 120.0 gr
Output Data
Recoil Velocity: 9.7 ft/s Recoil Energy: 12.4 ft•lbs
Recoil Impulse: 2.6 lb•s
In comparison the .280 British in an 8.5 pound rifle;
Charge Weight: 30.0 gr Muzzle Velocity: 2550.0 ft/s
Firearm Weight: 8.5 lb Bullet Weight: 140.0 gr
Output Data
Recoil Velocity: 8.4 ft/s Recoil Energy: 9.3 ft•lbs
Recoil Impulse: 2.2 lb•s
And
5.56x45 NATO M855A1 in a 7.5 pound rifle;
Charge Weight: 25.0 gr Muzzle Velocity: 3100.0 ft/s
Firearm Weight: 7.5 lb Bullet Weight: 62.0 gr
Output Data
Recoil Velocity: 5.9 ft/s Recoil Energy: 4.1 ft•lbs
Recoil Impulse: 1.4 lb•s
You see what they're fixing to do here, right?
They are reinventing a wheel that didn't work all that well the first time trading the current set of problems for the *old* set of problems. Once again they are fixating on a single factor amd are once again cueing up to miss the sweet spot in the middle where they'd find a good balance of recoil, trajectory and lethality.
Edit:
Another potential problem dates back to the adoption of the M16A2 and M855 rounds. In 1984, the USMC conducted a 6000 round test comparing 50,000 psi M193 55gr round to the 53,000 psi M855 62 gr round, which had boosted chamber pressure from 50,000 psi. The short version is that the M16A1s saw virtually no accuracy decline, while the M16A2s saw their groups more than double in size when the sole variable was chamber pressure.
The new 6.8mm round will probably be operating around 56,000 to 58,000 psi. In addition to potential accuracy issues due to the higher maximum average pressure, it'll pose additional challenges such as increased gas port wear, which in turn increases cyclic rate, which impacts reliability and durability when all other things are equal and cartridge pressure is lower.
Again, it's ground we've covered and mistakes we've made before.
)
