Also worth mentioning is the NIJ rating system. Your rifle plates are going to be rated as Level III (not to be confused with Level IIIA soft armor) or Level IV. Level II plates will be tested against 6 rds. The Level IV plates may be single hit or multi-hit rated (up to 6). They also might be stand alone or In Conjunction With, usually written as ICW, meaning for the rating to apply they need to be worn with a soft armor backer.

Per the NIJ testing protocol, a plate rated at Level III has to stop 147gr 7.62mm. Level IV has to defeat 166gr .30cal AP.

Your armor selection needs to be based on the threat. If your realistic threat level is high velocity rifle rounds, it might be worth level IV plates.

Notice 5.56 is not in the testing protocol. This is especially important given that AR500 steel isn't great against high velocity projectiles like M193 out of a 16in rifle. Once velocity goes north of 2900 FPS the round will ding the hell out of steel. Armor quality or otherwise.

If you're interested in this stuff look up Doc Gary Roberts posts on Lightfighter (he posts as DocGKR)

http://www.lightfighter.net/forum/he...and-body-armor

It might also be worth glancing through the NIJ stuff on their .gov website.

http://www.nij.gov/topics/technology...s/welcome.aspx