Centurian Arms, centurianarms.co_m
Made from billet, strong and accurate, I know this because I designed and programmed them for him. Yes, buffer tubes make a difference, the tolerance of the tube, also the air holes in the back make a huge difference in how they respond and work..
Stock is whatever you feel comfortable with, or find ascetically pleasing, the only thing that does it help you find a comfortable set.
He also makes the correct size for the AR10s as well if anyone is looking for those, which are not as common.
That's always been a weak point of the AR.