Luckily it's pretty easy to bore sight an AR. I normally go with 50 yards, but 25 works too. Just don't go too short with the distance!
The first time I bore sighted an AR I was in my basement. I thought I was being smart because I had it locked into a vise so it couldn't move. I put a small piece of masking tape on the window 7 yards away and aligned the scope with the bore, and headed to the range. My local rifle range starts at 50 yards and we aren't allowed to put up makeshift targets at shorter distances, so instead of starting there I went to one of our pistol bays that allows rifles. There was a target set at 7 yards, so I fired a shot and hit dead center. An older gentleman sighting in a shotgun didn't believe I was dead on after bore-sighting, so he demanded I shoot a 2nd round, which also hit dead center. Thinking I had things good, I went off to the rifle range and set a 16" x 22" target at 50 yards. And I missed. And I missed again. I threw several shots downrange, aiming at various squares on the target and never once saw an impact. Frustrated, I decided to bore sight again at 50 yards. This time I was within 2" of the target on the first shot.
Being so new to AR's at the time, I just didn't realize how much those high sights change the point of impact at various distances. After that 7 yard zero I was probably shooting 16" high at 50 yards, so even aiming near the bottom of the target my shots were hitting just above it.