I have a long range AR. I have a shilen select match, heavy profile, rifle length gas block, 1-8 twist, 20 inches, with a muzzle brake. It is a very accurate rifle. Around .75 MOA with Hornady 75 grain Match. I've had a couple of .5 MOA groups and a few .25 MOA with the same gun. I have taken it out to 750 yards consistently on a 10 inch plate, but I struggle to get past 750 with any kind of consistent accuracy. I have yet to hit an 800 yard plate with this rifle shooting 70s up to 77s. I think it's doable, but I don't think I'm capable with my current set up.
300-500 yards should be a cake walk.