Dude, a lot of cops wear IIa vests. I don't know where you got the idea they were obselete. They are simply the lowest level of ballistic protection, Level I vests being mostly fragmentation protection.
I'd be rather surprised if a department were still issuing Level IIA's for duty use. New ones aren't significantly cheaper than full NIJ Level II's, at least on the sites I looked at. From what I've read, IIIA seems to be the choice of departments that can afford it (or where the climate permits), and II the choice of those departments who don't want IIIA.
Perhaps describing IIA as "obsolete" was a poor choice of words. "Not widely issued by law enforcement agencies anymore" would probably be more accurate.
A IIa vest won't even stop a .357 magnum? That may be true, but a .357 magnum is a pretty significant pistol caliber.
It's also one of the more common calibers that police officers may face. The Bradyites are being deceptive when they paint the 5.7x28 with civilian ammunition as having mythic vest-busting powers, when they intentionally chose a vest not rated to stop the common .357. They are also not advocating a ban on .357's...at least not yet...
IIa will stop 9mm fmj round nose and hollowpoint (~1000 fps, .45 acp round nose and holllowpoint (~900 fps), and .40 S&W round nose and hollowpoint (~1000 fps) as well as lessor calibers such as .380, .38 Special, .32 acp, .25 acp, and some .22 lr. In short, the 'obselete' IIa vest will stop much of the common ammo found used by typical bad guys.
IIA is better than nothing, sure. So is Level I, which will stop up to .38 Special.
Your description of 12 ga slug ballistics with kevlar is amazing, naive, but amazing.
Perhaps you need to read up on NIJ body armor classification ratings and test protocols. A NIJ Level IIA vest is
not rated to stop a 12-gauge shotgun slug, and in fact the Bradyite video clearly shows the IIA vest failing the test via the NIJ backface deformation criteria. I refer you to National Institute of Justice Standard 0101.04, "Ballistic Resistance of Personal Body Armor," which classifies more than 1.73 inches of backface deformation (44 mm) as a failure, since that is enough to kill you. The Bradyite video clearly shows several inches of backface deformation, and that's not even taking into account temporary deformation of the backstop.
Here's a direct link to NIJ 0101.04 Rev A, if you don't believe me:
http://www.nlectc.org/pdffiles/0101.04RevA.pdf
NIJ 0101.04 Addendum B (minor clarification on test procedures):
http://www.nlectc.org/pdffiles/04AddendumB.pdf
The FN 5.7 round will penetrate Level II and IIIa vests just fine. Not only that, but it will do it without the need for the steel core, the illegal for civilians to own steel core AP ammo. The jacket lead core ammo goes through kevlar just fine.
How is this possible? Simple. Its pointy like military ball ammo that will zip right through a vest, even lead core FMJ. The kevlar works by esstentially 'catching' the incoming round in the weave and dispersing the energy over a fairly wide area as the kevlar is impacted by the bullet. This works great for most round nosed pistol ammo, shotgun pellets, etc. However, the kevlar weave is a poor performer at stopping stabbing implements like ice picks and stiletto knives. The FN 5.7 lead core FMJ round will pass through the kevlar just like an ice pick. Ice picks and pionty military ball-type ammo penetrates by using the point to pass between the fibers, thereby negating the fibers' ability to catch the incoming round.
Prove it. The Bureau of Alcohol, Tobacco, and Firearms says you're wrong.
Rifle ammunition penetrates Kevlar by overloading and breaking enough fibers to penetrate, not by slipping between them. Blunt softpoints will penetrate just as well; the key is velocity.
When a 9mm hits a vest, the fibers in the first layer decelerate the projectile and absorb its kinetic energy until they break. It's a war of attrition between the kinetic energy of the projectile and the energy absorption capability of the Kevlar. If you run out of Kevlar before the projectile runs out of kinetic energy, you have a penetration.
To slip between the fibers, you have to have a projectile material
hard enough to resist deformation under severe loading, i.e. the sharpened steel tip of an icepick. Federal law bans hard projectiles from handgun ammunition and .223, 7.62x39mm, and 7.62x51mm/.308 rifle ammunition, so that point is moot.
Note that when folks talk of armor piercing ammo, they are not referring to ammo that will pierce soft armor. They are talking about piercing hard armor.
When the prohibitionist lobby talks of armor piercing ammunition, they are often speaking of Kevlar-piercing, not military AP. That was the criterion by which Senator Kennedy called .30-30 Winchester "armor piercing" during a memorable floor debate during the 2004 session, if you recall.