To Quote them :
"A 2006 Joint Service Wound Ballistics-Integrated Product Team report showed the “clear and unequivocal best performing” cartridge tested was 6.8mm."
Now I would point out that the test done in 2006 showed the 6.8 to beat the other rounds overall, but this new .277" bore round didn't exist in that test, so I can't put any stock in that test. it was a kick in the crotch for the worshipers of the 6.5 Grendel, but that is how it shook out. It must be remembered that the test had certain perimeters most of which didn't revolve simply on ballistic flight characteristics or even energy.
The rounds had to fit in an M-16 magazine and M-4 type carbine.
The idea was to come up with a round that was better at killing or incapacitating enemies out to 400 yards better then the existing 5.56 NATO round.
Some rounds like the Grendel did "fly better" then the 6.8 SPC out past 400 meters but with the bullets available to test in that year the 6.8 out killed the 6.5 at every range when tested at Brag and Benning and later at Marine Corps Camps in Va. 5th Special Forces did a pretty extended deer hunt with the new rifles and that was kept away from the public for about 6 years to keep the libs from bitching about it.
The 6.5 MM rounds did shoot flatter and buck wind a little better but that was not what the Army was looking for. To quote General Krulak "we don't need a long rage rapier, we need a close to mid range hammer. Our current 7.62 and even our 5.56 already do good work at long range"
Another kick in the crotch for the 6.5 Grendel in 2006 was the nonavailability of good magazines that held the required number of rounds and still functioned at 100% reliability. The 6.8 mags they started with worked fine.
Now that the Army is thinking about a total replacement or both weapon and round, I think the 2006 test may be invalid.
What ever new round they pick need not fit an M-16 mag anymore so it seems.
The last rifle the USA adopted that was made in concert with the round before the AR-15 (re-named the M16) was the 1903 Springfield. The Springfield and the 30-03 and 3006 round were developed together. The original AR15 prototypes were made in 222 Remington.
The M-1 Garand was re-designed to use the 30-06. Garand originally made the rifle around the 275 Peterson round.
The M14 was a re-work of the basic M1 Garand idea, and the 308 winchester (7.62 NATO ) was simply a 30-06 shell with the shoulder moved back to the existing powder charge so the air space was eliminated.
But it may be that the new rifle is going to be designed at the same time as the new round. Like guns were made in the late 1800s and very early 1900s. That may be very interesting. Instead of a re-work of existing weapons, this time they may start at "ground zero" and simply make the whole system together. That requires a lot more money, but the result is likely to be superior.
Cool, wonder which generals have a financial interest in the company slated for supply.
