New military round. it has impressive velocity out of a 13 inch barrel, but at 80,000 psi chamber pressure. Below is another link where a guy talks about the cartridge.
No reloading dies yet for it. Sig makes a round that is ball ammo, that has an all brass case. Then a round that is for hunting that has the steel band at the bottom of the case. I see Midway has the FMJ for a box of 20 at $33.00
Its noted that at 80,000 psi barrel life wouldn't be very long which makes sense. Also brass wouldn't last very long.
In the video he mentions that the 277 has more than double the recoil of the .223, but the suppressor reduces that by 40%. Which makes it manageable for rapid fire.
https://www.youtube.com/watch?v=ALvd7k1Bbpw&t=324s