I don't think you'd ever exercise that 20x on a 5.56 rifle :\ IMO, you'd be better off running an ACOG (any model will do, doesn't have to be an NSN model) and a smaller MRD on an offset.

Another option: Have you ever heard of the Bindon Aiming Concept? It seems to be a trick that a lot of people haven't even heard of, despite the ACOG being designed to take advantage of it. The concept is while looking through an illuminated reticle magnified optic (it IS possible with a high magnification scope, but works best around 4x) with both eyes open, focusing on your target just like with a red dot, the illuminated reticle will float in your field of view superimposed over the target, in much the same fashion as a red dot functions. The image through the optic will blur out and fade, but the reticle will still appear as a bright spot. It's not AS quick as a red dot, but I've used it while airsofting (with a NCStar cheapy 4x light up reticle) and had a great deal of success with it. The main difference is a magnified optic won't be parallax free, so accuracy at more than 20-30 yards will require actually sighting through the optic and focusing your eye. But for bad breath distances, it's more than adequate, and can be very fast.