Quinc said:
...I was having no problems getting them to 1k with my 26" AR15. ...
Define "no problem" please.
Getting it to punch a hole somewhere on the target frame at 1000 yards, sure.
Shooting scores that are Master or better with a sling or Expert in F class on a weekly basis, not so much.
I've known one guy who shot a 223 at the HM level with a sling, and he was running 90VLDs out of a match rifle with 30" barrels at velocities and pressures that trashed the brass pretty fast. I've never seen anyone shoot the 223 from 1000 in F-TR and do it consistently well. About the only masochist I know who keeps trying is Bruce from Canada.
At 600 yards, have a blast with it, heck, come out and shoot it at 1000 and have a good time, but recognize that beyond 600 you are handicapping yourself.