what is an armor piercing bullet?

[Lizzie Poppet]

3. Technically, none of those rounds are "armor piercing."

Most of those you list, if using a fully-jacketed bullet, will probably defeat lower-level body armor (most any hunting rifle caliber will do so, but those pistol rounds woudl be iffy, even the .44 mag), but aren't specifically designed for penetrating armor. Ammunition designed to defeat armor will have a steel (or tungsten, beryllium, etc.) core in the bullet, or even a sharply-tapered bullet entirely of the harder metal.

There are sensible federal restrictions on armor piercing rounds for handguns (essentially, you can't buy such ammo), designed to keep criminals from having ammo that will defeat police body armor. Criminals rarely use rifles, and the handguns chambered for rifle rounds have the same problem for criminals that rifles do: they're far too bulky to conceal.