Jimro, regarding the Army's use of AR10's breaking records, check the "New 1K Service Rifle Record" part in:
They got the rules changed in 2009 so they could shoot better scores than what some 7.62 NATO Garands and M1A's were shooting at the long range matches at the Nationals outscoring what the 5.56 ammo produced. No M14NM's were in inventory that could do that. The load for the AR10's was approximately 45 gr. of Varget with the Berger 185 LR bullet. That's normally the max load of Varget under a 155 so I can imagine what the peak pressure is. That bullet needs at least 2550 fps to do well at 1000 and the AR10's barrel is only 20" long.
Also note those Redfield Olympic (like their International) rear sights were notorious for slop as elevation increased. Redfield's Palma rear sight was better. But none of them had enough windage to correct for winds more than 20 mph when shooting the .308's at 1000 yards. 20 MOA don't cut the mustard which is why the Clerke, Wilkes (from England), DeFina and later Warner rear sights with at least 30 MOA of windage either side of zero were preferred as well as having near zero slop and backlash in their adjustments.