GBullet is absolutely right about the clay-dent approach; that's how they actually "determine" the rating of a given vest. Not whether the bullet actually penetrates it, but how much "backface deformation" there is in the clay when shot by a given load at a given vest. Don't recall what the maximum indentation to "pass" is, but I believe it's something like 2 - 2 1/2 inches. (Sounds like a big "dent" in your chest, but remember that the Red Cross teaches to use 3 1/2" of compression when doing adult cpr, so it's less than that...)
Say a load doesn't penetrate a vest, and leaves a 2" dent in the clay behind it, the vest is rated for that load. If it doesn't penetrate, and leaves a 3" dent in the clay (assuming I'm remembering the 2.5" rule correctly), it FAILS the test, even though it stops the bullet, and the dent is still shallower than a cpr application. The vest would then NOT be rated for that load.
That female trooper story I mentioned - her vest "failed" that government-decreed test, even though it not only saved her life from the bullet, it actually kept her in good enough shape to stay in the fight.
That's been one of the complaints about vest ratings for years; that bureaucrats won't purchase or allow vests that aren't "rated" for a certain level, and to meet this arbitrary "level" (that has little bearing on effectiveness) the vests have to be so thick, stiff, and cumbersome to meet these ratings, that many people won't wear them. I know several guys who carry theirs in the back seat and say they'll "grab it if they need it". My response is that "since you'll need it anytime you need your gun, why not leave your gun in the back seat as well?" (Can't understand why I piss people off...
The point of this rambling is that even if a vest isn't "rated" for a given load doesn't mean it's defenseless against it. It's a relative scale, based on an arbitrary testing formula that was originated by a bureaucrat with his kid's Play-Doh. (I kid you not; he chouldn't think of any other way to test a vest's function, so that's where this "backface deformation" premise came from...)
[/rant off] :duck: