This is a bespoke cartridge that Sig made specifically for this weapon and this competition. SAAMI has standardized it as the .277 Fury but Sig calls the (same) cartridge they're selling to the military the NGSW 6.8x51.
And its max chamber pressure is 80,000 psi. Yes, you read that right, eighty thousand psi. Which to the best of my knowledge is 15,000 psi and 19% hotter than any other factory small arms cartridge in existence.
It is SO HOT, in fact, that you can't expect a conventional brass (C26000) cartridge casing to endure the pressure. So they've made the brass two-piece, a steel case head mated to a brass body:
At left, the .277 Fury with steel case head
And all that pressure is so it can drive a 135-gr AP bullet to 3000 fps MV from a 16-inch barrel. Which is muzzle energy on par with a 7.62x51 NATO firing a 175-gr bullet from a 24-inch barrel.
And since it's a smaller diameter bullet than the 7.62, it has a better sectional density and a higher ballistic coefficient, too, which means both its range and downrange energy are superior to the 7.62 NATO cartridge, despite the 6.8 having only 2/3rds the barrel length.
But this is only a stopgap until they have selected a general issue infantry rifle that will fire, if not the exact same cartridge, then one with similar capabilities. Because Big Army's idea is that in future wars everybody will be wearing body armor. And if you're standard infantry rifle won't defeat the enemy's body armor, you might as well be carrying a Brown Bess.
And the .277 Fury/NGSW 6.8x51 is believed capable of defeating, not just all current body armor, but any IBA that anyone has on the drawing board.
With that much chamber pressure you can be sure this thing eats barrels for breakfast (so did the much-vaunted MG-42), but Sig delivered the prototypes with 825,000 rounds of ammunition. With that many rounds downrange, even the army couldn't have missed the fact that this thing has a hearty appetite for barrels, and they've consigned themselves to living with that fact.
So carp all you want, this is the future if infantry armament.