Second thing: For most modern combat, 300 meters is plenty far. I carried an M14EBR (Enhanced Battle Rifle) in Afghanistan, and I could consistently hit a torso-sized rock at 900 meters – at the range, with perfect weather conditions, a good firing position, on a stationary target at a known distance. In combat, with extreme heat or cold, unknown distances, hasty firing positions, adrenaline and moving targets, plus little annoyances like incoming fire, I would have been ecstatic to smoke a mofo at 200 meters.Every now and again, the stars will align, and a (usually very good) shooter will get a chance to hit targets in combat beyond 500 meters, but this is very rare. Even so, specialized 5.56mm ammunition seems to perform decently at these ranges.
Go forth and read the whole article (which is about optics selection).