what is an armor piercing bullet?

[TexasProgresive]

2. Evidently bullets with a steel, iron or brass core-certainly not hollow points of any kind.

US considers ban on type of ammo that can pierce vests
Obama administration considering ban on type of ammo used in AR-15-style rifles because the bullets can pierce a cop's armor when fired from a handgun

Mar 3, 2015
http://www.policeone.com/police-products/body-armor/articles/8376889-US-considers-ban-on-type-of-ammo-that-can-pierce-vests/
An ATF spokeswoman, Ginger Colbrun, said Monday the agency is considering eliminating the exemption now because of the production of so-called AR pistols that can fire the same cartridge. The agency is accepting public comment about the proposed change until March 16 at the email address [e-mail], by fax or postal mail. Colbrun said it's unclear when a final decision will be made.

At issue is the material in the core of the bullets. As long as the bullet's core does not contain particular types of metal, including steel, iron or brass, the bullet would still be legally available.