What makes a sub-MOA 6.8?

So, I'm looking at the 6.8's and 6.5's right now for my next upper. Coming into this, I thought I was a surefire 6.5 buyer. The info on this site, though, is definitely making me step back and really examine the two cartridges.

Demand: MOA to 600yds
Want: 1/2 MOA to 600yds

After looking back and forth, comparing ballistics, throat erosion and what not, I'm still left with a fundamental question: How do these two rounds compare on a 600yd MOA basis. Actually, the question is a quite a bit more general than that: Any rifle I buy I want to be as accurate as possible. Stripping out ammunition, shooter and environmental factors, how do I evaluate one to be any more inherently/mechanically accurate than another without either a guarantee by the manufacturer (that can be acted on) or real world tests?

Truth is I am looking for a < 300yd deer rifle that will serve secondary duty as a varmint and target rifle. No issues with the first, pick the upper of your choice and you're taken care of. The issue lies, however, in that I am also an accuracy nut and like to shoot out as far as I can for varmints and paper. At that point, you can impress me all you want with ballistics, but if the round is hitting at lightning fasts speeds but turning in 3-4 MOA groups, I just don't care. The chart here makes me think of just how disappointed I would be in all but the MSTN and Noveske rifles on that chart

So, what goes into building a sub-MOA AR in 6.8 (or any other caliber for that matter)? What should I be looking for specifically to ensure the highest accuracy possible? I thought it was primarily the barrel, but that chart makes me think there must be a host of factors.

Any input would be appreciated,
Pop