what is an armor piercing bullet?

[Travis_0004]

4. According to the ATF

18 U.S.C., § 921(a)(17)(B)

A projectile or projectile core which may be used in a handgun and which is constructed entirely (excluding the presence of traces of other substances) from one or a combination of tungsten alloys, steel, iron, brass, bronze, beryllium copper, or depleted uranium; or

A full jacketed projectile larger than .22 caliber designed and intended for use in a handgun and whose jacket has a weight of more than 25 percent of the total weight of the projectile.
The term “armor piercing ammunition” does not include shotgun shot required by Federal or State environmental or game regulations for hunting purposes, a frangible projectile designed for target shooting, a projectile which the Attorney General finds is primarily intended to be used for sporting purposes, or any other projectile or projectile core which the Attorney General finds is intended to be used for industrial purposes, including a charge used in an oil and gas well perforating device.

Of course all armor has a rating. Most bulletproof vest wont stop a rifle round. Light armor designed to stop .223 will not stop a .308. In that sense .308 will go through a lot of armor, but its not armor piercing.