What is the ideal range for sighting in a red dot sight on a AR15?

I sighted my AR for 25 meters (81 feet (and it hits dead on)) which military says if you sight a m16 or m4 at this range it should hit at 300 meters..... now I did this but is it different for a red dot sight? And when i shoot at 50 meters my rounds hit a little up, is this because of the bullet projectile?