Q. That's really complicated. Simple question: Why is M193 better than M855?
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.
Counter Opinion: 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.
Counter Opinion: 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.
Fact: Some penetration stats for M193 and 7.62 (M80): Thickness of material for positive protection against caliber ammo listed. Concrete (5,000 psi), 5.56: .5 inch, 7.62 and 30 cal, 7 inches. Wet sand, 5.56: 25 inches, 7.62 and .30, 36 inches. Packed or tamped earth, 5.56: 32 inches, 7.62 and .30: 48 inches.