: Projectiles made entirely (except for a core) of materials like steel, brass, or tungsten alloys.
According to the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF), 9mm ammunition is classified as AP based on its construction: buy armor piercing 9mm ammo
makes it illegal for civilians to purchase, import, or manufacture armor-piercing ammo. Ammunition Regulation - Giffords.org : Projectiles made entirely (except for a core)