"I have seen fmj .223 bore a clean hole in 1/4" steel plate, and eat up a brick wall behind it. I don't think 9mm will do that!"
No, but the ammo that will go through plate and brick is hopefully not the same ammo an entry team is using. I would imagine they are using somethink light and frangible (sp?).
barnes has a bullet out right now called the 'varmint gernade" It fully franges going through a grape.
As for why a vest is rated for 9mm or 5.56 is that a 5.56 is much pointier, and the projectile (even a frangible one) will "needle" through the lower rated vest moreso than a blunt 9mm or 45 cal. Once through the frangible round begins breaking down.
This was part of the design consideration for FN's five-seven AP rounds.