I'm not sure what you mean by "cores" but level III plates are rated to stop 7.62x51mm M80 FMJ at 2,750 fps or less. Unless you use some very high velocity .308, it is not getting through. As for the fragmentation coating, the plates are offered with a buildup coating as seen in the video, which is very effective. There is another video on my channel where I shoot this same plate ten times with Norinco steel core 7.62x39mm and it contains all the fragments. You are also mistaken about the degree to which those fragments will injure. A couple years back, I laid a gelatin block next to an AR500 target and shot it several times with Yugoslavian M67. Without any protection at all (remember gelatin blocks don't have skin), the fragments mostly penetrated about a half inch, with some larger fragments getting about 2 1/2" deep. With two layers of denim, which is far less cut resistant than the nylon that plate carriers are made from, the deepest fragments only went about 1 1/2" in. This is enough to cause a very painful wound, but unless you are very unlucky and take a fragment at exactly the right angle to get your brachial or carotid arteries, the fragments are unlikely to be life threatening.