As an update.... I read an article about the Lake City ammunition plant that supplies nearly all our military ammo.... as we speak, they are adding a 450,000 sq ft facility that is specifically for the manufacturing of the new 6.8 x 51 ammo. They will make everything in-house.
The report says that SIG will supply our military with loaded ammo for about 2-3 years, until the LC facility is up and running.
So, this cartridge/rifle changeover is a very real thing. The report says this is the first "new" cartridge that the LC facility has loaded in more than 65 years.
This cartridge is essentially, ballistically, a 270 Weatherby Magnum, in a shorter action, autoloading rifle.