It is important to know that Level II-A, II and III-A all stop the overwhelming majority of pistol projectiles you are ever likely to encounter (plus 12 gauge, OO buckshot), and also to know that NO armor is ever 100% ‘bulletproof’ under ALL conceivable circumstances.
However, these ratings often have a safety margin for penetration because blunt trauma is usually the limiting factor in certification. For example, Level II body armor would likely stop the III-A test standard, (9 mm submachine gun at 1400 fps) from actually PENETRATING through the Level II vest. But, the Level II vest would fail on blunt trauma impact (the NIJ deems any dent greater than ~1.7" (~44 mm.) on the soft clay test surface, a FAIL).
So, the advantage in increasing protection Levels from II-A, to II, to III-A, is NOT so much protection from PENETRATION of pistol fire, but a significant reduction in the blunt trauma received.
