In a nutshell: Advantages of M193 over M855:
Though it isn't a bad idea to keep a couple of magazines worth of M855 in case you need to make a long-range (300+ yards) shot against a "hard" target (a vehicle or other equipment), most folks are better served with M193 for general use.
Many optics (like some of the full size ACOGs) are calibrated to the ballistics of the 62gr M855 round, not the 55gr M193 rounds. Granted this is only a slight difference inside of 300 meters, but it will compromise the zero at longer ranges.
Keeping two kinds of ammo is rather foolish. It just leaves the opportunity open for confusion during a crisis, when it's most important to keep things simple.
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