I'm pretty sure that they are spec 2 chambers. The only thing negative I've heard about the Stag uppers is they don't cut M4 feed ramps into them, which the 6.8 needs. If you're buying ammo, get it from Silver State Armory, they make the best ammo and the easiest brass to reload. They also sell their brass.
Best load accuracy wise is 90gr TNTs, 29.5gr Reloder 7, SSA Brass, Federal Match primers. I'm getting sub 1/2" groups with those. Hunting, 29.5gr H322, either 110gr Sierra Pro Hunters or Hornady Accubonds. I get 0.6" groups with the Hornadys, about 1" groups with the Sierras.
BTW, head to
..::68FORUMS::.. for a lot of good info specifically on the 6.8.
Jim