As sstated, it is a 6.8x43 chamber. Ir is more accurate than spec II, PLUS, YOU CAN LOAD HOTTER THAN STANDARD SPEC II chambers. The barrel is melonite finished. Radius, 5 groove. You will average around so 50fpsmany more than most barrels even with mild factory loads. That barrel will actually yield 20 inch velocities all day.
That is the same setup I have been shooting for a few years.
Harrison is 6.8. Without him it wouldn't be here the way it is. He, TimW, HTR, and a couple of others resurrected this thing and made it what it is now, which is what is was intended to begin with. Remington reversesd. Some numbers on the blueprints and fucked up the chamber, wound up downloading the ammo, and then rather than fix it, tried to let it die, and has tried to kill it.
Stay away from anything Remington in 6.8, ammo, everything, it all sucks, and is overpriced. Brass is fair, but large primer, and you want small primmed SSA or Hornady.
You won't find a more consistently accurate upper.
Harrison at that time, trued the receivers on every model, even the cheapest, which this was not. This is the recon package.
Most have Mega upper receivers, check and see for sure. But mine does.