The 6.8 has 2 different chamberings, one is the older style that had pressure problems and the newer spec II (I think) is for the hotter loads. Depends on barrel twist as well.
Remingtn brass also used the large primer while Hornady brass used a small primer, allowing more pressure.
This is off the top of my head from when I was interested in the round so I maybe wrong on a few points.
