"Cop killer bullets" -- in 1936
A research find, from the papers of Homer Cummings, FDR's Attorney General. J. Edgar Hoover (then signing as John Edgar Hoover), writes that S&W is going to produce a ".357 Magnum," a round plainly meant as a cop killer. It can't be meant to increase stopping power, he adds, because we all know that slow, heavy bullets are the key to stopping power. (This actually was a common belief at the time, that low velocity somehow increased stopping power).
UPDATE: zinc bullets were still an issue in the 1970s (i.e., before Super Vel brought out the first functional expanding pistol bullets). The idea was to cast a bullet out of zinc alloys that were of the right hardness to take the rifling without damaging it. Zinc is much less dense than lead, so they could be pushed to much higher velocities. In the days before Super Vel, a light nonexpanding bullet at high velocity had potential for increasing stopping power. I was surprised to find that this issue had been raised in the 1930s!