From the National Institute of Justice (NIJ) website:
5.56mm ammunition designed for military use, with a steel or partial steel core, needs a ceramic or metallic component to the hard armor plate to stop it. Some Level III have a ceramic/metallic face, but not all. If you have identified the M855 or other similarly designed steel core ammunition as a potential threat, a Level IV plate is the best choice for protection among the armors on the Compliant Products List (CPL).
NIJ LEVEL IV 10X12 SAPI CUT SINGLE CURVE CERAMIC ARMOR PLATE FEATURES:
SAPI Cut Single Curve.
NIJ 0101.06 Level IV.
Protection from 5.56mm green tip SS109.
Mil-Spec Ceramic Core
Waterproof Polyurea Coating for lasting Ballistic Integrity.
Weight 6.3 lbs.