They should have went with the NATO .308 or 7.65 x 51 round.
I love the .308, but my AR10 has a 20 round vs. 30 rd mag for my AR15s and I’m pretty sure they weigh the same.
Sometimes quantity has a quality all its own.
Reading about the 6mm ARC made it sound like a good choice, can use the 5.56mm size receivers. Better ballistics, normal chamber pressures, etc.
The 7.62 just has too much of a weight penalty. Running with five 20 round magazines would beat you to death slapping around. Full auto on anything of reasonable weight is almost not controllable.
I love my AR10 build, but would not want to carry it around for long.