AR500 plate will easily stop M855 ammo. In fact, M855 pocks the plate LESS than M193 at 100 yards or closer.
I did a test of M855 and M193 out of a 20" AR15 once. I shot AR450 plate at about 50 yards. The pock marks on the plate were roughly 30-40% deeper with the M193. M193 is moving about 200 fps faster.
As the knowledgeable Steel Target makers will tell you.... VELOCITY does the damage.