The rear sight on the M16A2 (and the ARs that have the same carrying handle) is made so you can dial in a distant range temporaraly (300 - 800 meters). Out to 300 meters, you simply use your iron sights while being aware of the difference between your line of sight and the ballistic path of the bullet. Also, that assumes you are using the army 25m/300m zero method. If you are using a 50 yd or 100 yd zero, that dial on the carrying handle is pretty useless. In the civilian world, I think the common wisdom is that if you see a need to shoot that far, you would be better served using a decent set of optics.
If you need to have that distance in a set of iron sights, perhaps a rear sight (cut down from a carrying handle) or a Matech sight would serve you better than a MBUS.