Just something to keep in mind for this discussion. There are 2 types of AP ammo. The first type is the legal definition that classifies AP ammo on the basis of it composition. Here is a good link describing this...
http://yarchive.net/gun/politics/armorpiercing.html
The second type of AP ammo is ammo that is not made with the compositions noted in the legal definition, yet still manage to penetrate armor.
Also keep in mind that when folks talk about armor, there are two categories of armor, soft and hard as pertains to armor worn on the body.
Soft armor includes materials such as kevlar. There are different levels of kevlar armor ranging from I-IIIA. IIIA is the highest rated soft armor and it will stop the most types of rounds of soft armor. Each level represents the ability to stop certain types of ammo plus will stop anything also stopped by lower rated kevlar levels.
Hard armor is just that, hard. It may be made from ceramic, steel or other hard material. There are two levels, III (high power rifle) and IV (armor piercing rifle).
While ballistic vests are rated by the ability to stop various projectiles, AP ammo is not rated in regard to what type of armor (composition, thickness, hardness) it can penetrate.
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In regard to SB88LX's female's story about her father having .45 acp AP ammo, keep in mind that a lot of ignorant gun and ammo folks think that hollowpoint ammo is made to penetrate soft armor. I can recall seeing an episode on Cops where a gun was arrested and a pistol taken from him. The New Jersey officer dropped the mag and looked at the ammo. It was hollowpoint ammo and he said it was designed to go through police vests and was a cop-killer ammo.
In reality, hollowpoint pistol ammo is less likely to penetrate soft armor than regular ball ammo. This is because on strking the soft armor, the sides of the hollowpoint collapse, pancaking, there my reducing the size of the hollow and making the bullet much wider. This all takes place on the exterior of the vest, with little or no penetration of the kevlar.