In the '70s a company called KTW introduced a tungsten alloy bullet which was quite hard, and designed to penetrate metal. Marketed ONLY to police, the bullets had a Teflon coating to engage the rifling - the tungsten alloy was too hard to be engraved by the rifling, hence the teflon "jacket."
Tungsten got too expensive, so I believe the company modified the bullets to a bronze alloy.
The media found that these bullets would penetrate Kevlar vests, and so called them "Cop Killers." Of course, their hysterical reporting served only to bring them to the attention of bad guys . . . and anecdotal reports had bad guys trying to shoot cops in the head, which wasn't protected by body armor. (Thanks, NBC!
)Of course, "metal penetrating" handgun bullets are nothing new - they've been around longer than I have. In fact, my 1947 "Western Ammuntion Handbook" devotes a couple of pages to their metal penetrating loads, meant for use by police when shooting at bad guys in getaway cars. I suspect at least the .357 loads would penetrate some Kevlar vests. (I'd guess the .30 Mauser round, dating to the late 1800's, probably would, too . . . and that wasn't specifically designed as metal piercing.)