They're building ARs now in the 7.62 NATO, accurized, for secondary weapons for the Sniper teams. Heard this from the horse's mouth. It's replacing the designated marksman 5.56 they formerlly carried. Ammo and shooting to the point of aim is the issue, I was led to believe.
Back in the day, I went to a US Army Counter-sniper Instructor Course. We shot the M-14 (I think they were called M 24 in Sniper configuration.) This was at the USAMTU at Benning. I was a cop and a SWAT sniper then. They no longer offer the course to civilians because of PC (either politically correct or posse commatatus, you choose) reasons.
It's relatively easy to get hits on a steel targt (steel so you can see the hit with a spotting scope) at 900 yards. This is from a foxhole, sling supported, with no danger of some skag spotting you first from a concealed position at 300 yards and drilling you. And, were I a sniper, and there was no wind and I had a clear targt, I'd probably take one. But not at the risk of revealing my position, because the chances are pretty dang low of getting a combat hit at that range.
It would require an environment without dust, dirt, and after a good night's sleep. While longer shots have certainly been made, one phenominal shot of something like 1200 meters, they depend on everything being perfect, plus a lot of luck. And that's why you hear about them.
On a good day, a proficient sniper could probably get hits all day long at 600 yards, but there are a lot of things that keep it from being a good day.
Also, from the same source of info on the AR 10-types, who was head of the Army Marksmanship Unit, a designated marksman with an M 16 (built) had confiremed 67 kills. This was two years ago. You don't hear about him, probably because most of his shots were from 150 yards or less.
