The following pictures illustrate perfectly the issue with the "frag coating" on most steel plates. There is a reason why we CHOOSE to not carry steel plates, despite having the ability to. The single base coats (1mm thick) are generally completely useless to stop fragmentation. The single coating is actually meant for rust/ corrosion protection of the underlying steel. Thicker coatings/ build up coats will often work for the first 2-3 shots (If they are smaller rounds such as 5.56, or at least 4 inches from the edge). Otherwise, you will run into issues of delamination. Bullet fragmentation follows along the strike face of the plate, and comes out the SIDES/ edges of the plate. The coatings are spray on coatings and will not bond to the strike face as strong as dedicated adhesives. The fragmentation will cause the coating to separate from the plate, and allow for the fragmentation to travel with little resistance outwards from the point of impact. This is what we call, delamination. The thickness on the front doesn't matter as much as the thickness on the edges of the plate, as that is where the most dangerous fragmentation will exit the plate. Near complete separation of the coating:
Here is a visual on what happens when a softer projectile hits a steel plate. Now imagine this happening under a frag coating, imagine what would happen when these high velocity hot metal shards hit the edges of the coating on the sides where it is the thinnest and weakest.
One of the main concerns is larger rounds (such as .308 M80 ball) hitting within 2-3 inches of the edge. Because of the larger round with more mass, often a single shot is all it takes for the bullet fragmentation to delaminate that localized section of the plate. As shown in this picture, a single shot of .308 within 2-3 inches from the edge was enough to cause separation of the coating, letting fragmentation exit the sides of the plate. As shown here, these were two .308 M80 ball rounds that struck around 2-3 inches away from the edge.
Put on your carrier, get into a shooting position and note what parts of your body would be hit by fragmentation coming out the sides.
Note parts of your body that are parallel to the plane of the strike face. (Neck, Chin, Arms, Legs, Groin, etc)
Remember, fragmentation from rifle rounds absolutely travel at enough velocity to penetrate plate carrier fabric AND tear into veins & arteries.
Pictures courtesy of Buffman - RANGE
He has a great video showing how bullet fragmentation from steel plates can kill.