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I am buying a 6.8mm upper for my AR here in two weeks
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What chamber dimensions are you going to be using? Here is a quote from another site concerning the 6.8 chamber dimensions.
Remingtons chamber drawings had sharp land shoulders & put the lands too close to the bullet. The result was too much pressure, so they downgraded the ammo from 2800 fps to 2650 fps, loosing @ 250 ft lbs of energy. Silver State Armory designed/redisigned a reamer that cuts the lands back & angles them like the original design. the reamer is made by Pacific Tool in White City, Oregon and is available for about $80. In talking with Pacific tool I found that Shaw barrels has changed over all their 6.8 spc barrels to the new SSA reamer design & Pacific Tool has supplied the reamers. Shaw makes a good percentage of all AR barrels for a lot of the major AR makers. Pacific also told me that it would be possible to cut the chamber with th reamer while the barrel is still in the receiver, that may or may not be the case, but in any event you are not changing the headspace or shoulder (like an ackley) just the throat land area. SSA has a combat load that they only recommend shooting in a re-reamed barrel that is pretty close to 2700 - 2750 fps out of an AR, depending on barrel length. Cardinal Armory has taken it a bit further and has reduced the twist rate and taken one of the lands out of the barrel for less resistance, they are getting about what Remington claimed at first 2800 fps, but out of 16-18" barrels with handloads! That's with 115 grain bullets. Using 90 grainers is putting it around 2900-2950 fps if I remember correctly.