It is EP (enhanced penetrating) over standard 55gr ball, but that's ALL it is.
What this means is the M855 penetrates better then the M193 (55gr ball) but does NOT penetrate enough to meet the military standards for armor piercing ammunition.
FORGET ABOUT VESTS!!!! Vests (what will and won't get through "standard" vests) are a smokescreen. It has NOTHING to do with AP ammo, or the current issue over re-classifying the M855 round.
EVERY common rifle round will punch through the "standard" soft vests without even noticing they are there. ALL the regular sporting soft points intended for big game will do it easily. Some of the smallest rounds loaded with varmint bullets might not, but loaded with solids, will do the same thing.
Going by the laws involved, what will go though what vest isn't even germane to the subject. it's a political smoke screen, the reason claimed by the gun banners for WHY they need to ban AP ammo. BUT, its NOT in any of the laws about AP ammo, anywhere.
In many ways, it is the same tactic used during the "assault weapon" debate. It has nothing to do with what the gun (or ammo) actually does, its all about what it looks like it does, or what they can convince people it does.
The military definition of AP ammo is based on what it will penetrate. The definition in Fed law of what is AP ammo is not. It defines AP based on what the bullet is made of, NOT what it does.
A bullet meeting their definition of AP (based on its construction) could be fired at a whole 25fps (twenty five feet per second -to be clear) and it would still be AP under the law. The fact that it won't penetrate a pop can at 30 feet doesn't matter at all.
Likewise, if you could fire a solid lead slug at 4000fps, and penetrate 16 vests stacked in a row, it would NOT be AP under the law. (and you can drive lead slugs to 2200fps which will swiss cheese any regular vest. A .30-30 can do this.)
Essentially, the Feds created an arbitrary standard, having nothing to do with the actual performance of ammo, to define AP. For over 30 years, they said the M855 ammo was not AP.
Now, with NO CHANGE to the LAW, and NO CHANGE to the AMMO, the Fed is trying to say M855 is AP.
Some of us just don't think that's right.