Re: DPMS lr 308
6" groups with a rack-grade LR-308 at 800yds is UNLIKELY. Not impossible, but unlikely. Again, I am not bashing on the LR-308, but most of them I have fired have been MOA or barely sub-MOA capable to 600yds.
Getting a semi-auto (or any rifle) sub-MOA accurate at that kind of range has as much to do (or more to do) with the shooter as it does with the equipment.
Run a couple of searches here (with Google) on the LR-308 rifles, long-range accuracy with them, custom LR-308 rifles and other semi-auto platforms for more information.