Q. But doesn't M855 penetrate hard objects better than M193?
It depends. At close ranges (up to 40-50 yards), M193 will penetrate thicker steel than M855, due to its increased velocity. Beyond 100 yards, M855's bullet construction starts paying off and it will penetrate better than M193. M855 also loses velocity more slowly than M193, with a cross-over point between 200 and 300 yards, beyond which M855 will have more velocity than M193.
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.