http://www.gunbroker.com/item/564189661

Interesting to read what they built although most of it is not necessary. I've read a lot about equipment used in NRA highpower rifle competition which typically fires from 200 yards to 600 yards. The standard of accuracy for an AR, an actual A2 configuration AR with open sights and 20" barrel and round plastic handguard, is a 10 shot group at 600 yards measuring 4" wide x 6" high. The 10-ring on the 600 yard target isn't much bigger than that. All it takes to get a competition AR to that point or close to it is a proper quality barrel, good chamber, and free-floated handguard. Fancy triggers and receivers and bolt parts might be prettier and last longer but won't improve accuracy.

The military teams pioneered the use of the AR in competition since it is after all called the "service rifle" class and the teams were using guns that hadn't been issued to troops in a long time. Once they discovered that they could use heavier bullets and get ballistics similar to the 7.62 match ammo that opened the door to widespread use in competition. As I understand now, people use 69 grain to 77 grain bullets for the 200 yard and 300 yard stages. That ammo has to be of proper length to feed through the magazine. For the 600 yard stage they use different bullets with longer points to achieve better ballistic coefficient, but the loaded rounds are too long to work through the magazine.

I don't doubt the claims in the article too much because I don't have the experience to compare with. I have read that the 80 grain bullets will maintain supersonic velocity beyond 600 yards out of a 20" barrel. How much further I don't know but I think they do have competition out to maybe 1000 yards. I think a 24" or 26" barrel would provide a significant advantage but would be more difficult to maneuver with. I wonder if the stubbier 77 grain bullet out of the shorter barrel will remain supersonic to 1100 yards. I mention supersonic because the bullets start to deviate a bit as they drop below the supersonic barrier so you don't want that to happen before the bullet gets to the target. I've read that 50 grain bullets become subsonic long before 600 yards and can become very inaccurate. Some ranges prohibit their use at the 600 yard line.

Back in the early days of the M16 I recall someone had hung up a large piece of canvas with an aiming point on it and fired at it from 1000 yards. The 50 grain bullets wouldn't maintain enough velocity to penetrate the canvas.