I've been down this road with ARs at longer ranges, furthest out I've gone is 650 yards. But...at that distance, I've had my best results with a purposefully built AR for accuracy: White Oak Armory 24" barrel, Geissele match trigger, match grade ammo (77gr SMK - Federal and/or Hornady), and a fairly powerful scope x20 magnification. I was able consistently to put rounds on a 12" plate and get it swinging back and forth with shots fired fairly rapidly.
With a regular M4 style AR, with regular factory 16" barrel, 16" etc. etc. You might find shooting at 500 yards with it a bit frustrating, since most AR's are going to not do much better than 2 MOA with regular mil spec ammo, so your spread at 500 will be 10" or more, plus factor in the challenge of a mil-spec trigger, etc. etc. etc
But with that in mind, you might want to consider a Primary Arms scope for your AR, they have received great reviews, I've owned a couple and they are just fine. They would give you the ability to reach out to 500 yards without breaking your bank.
Just some thoughts.