SIG-Sauer, NOT an American company no matter how many facilities they have in the USA, has convinced the Army that they have the special sauce to make their new rifle.
That includes ammunition.
6.8x51mm. A 135gr, .277 caliber bullet lobbing at [not specified] velocity from a 13.5" barrel from an 80k psi chamber. If you fire it from a non-NFA 16" barrel you get about 3,000 fps.
That's about the same as everyday common .270 Winchester in a 24" barrel at the MUCH lower chamber pressure of 65k psi.
Willard immediately noticed that 6.8x51mm is, in basic strokes, a necked down 7.62x51mm round.
7.62 NATO runs an even lower pressure than .270, a mere 60.2k psi.
Which made us wonder... Did we need to develop a whole new round to do this?
I'm not sure, yet.
First elephant in the room is the pressure is likely due to the need to get acceptable performance from the round from the 13.5" barrel. That length is almost definitely determined by keeping the overall length of the gun manageable with the standard issue sound suppressor.
The numbers for that acceptable performance are not jumping out at me searching the intertubes, but...
I'm finding nothing about what the XM5 does at the muzzle.
There's nothing earth shattering about the 16" performance other than it doing it 8" earlier than every other round doing it. That's because of the pressure.
I am very skeptical of the 12k barrel lifespan with those chamber pressures.
I am not skeptical of SIG being able to deliver the hybrid cases cheaply. I can think of a couple of ways to make them from the cut-away models they've shown that would be economical.