From Dave Kopel’s book, Aiming for Liberty: The Past, Present, And Future of Freedom and Self-Defense, Chapter Four, “Some Deceptions and Errors Used to Promote Anti-Gun Laws”:
The story of the nonexistent “cop-killer” bullet begins in 1976 in Massachusetts, when a handgun-confiscation initiative was defeated in a landslide. Then in 1982 in California, a handgun “Freeze” initiative also lost overwhelmingly. The gun-pohibitiation lobbies began to realize that they wold have to work more incrementally, rather than pushing for prohibition outright.
The prohibition lobbies also realized that the police were one of their worst problems. Wile a few police chiefs or sheriffs could always be found to support prohibition, the vast majority of police — both commanders and line officers — were extremely skeptical of gun control. Something had to be done to turn the police (or at least their Washington lobbyists) against the National Rifle Association.
The something, ironically, was an obscure type of ammunition invented by police officers two decades before. These bullets were known as KTW bullets, after the initials of the three persons who invented them: Dr. Paul Kopsch, and police officers Daniel Turcus and Don Ward.
The chapter goes on to describe what these so-called cop-killer bullets were actually created for, which was shooting through barriers. It also mentions their appearance in the Lethal Weapon series of movies, which is probably responsible for driving public misconceptions about guns during the late 80s and early 90s than a lot of others.
The problem with this gambit is that it worked, at least for a while. The anti-gun groups’ ability to drive misconceptions and public opinion on the gun issue used to be a powerful weapon against us, but it’s diminished quite a bit in the past decade. As much as I’d like to give new media the credit, I think it comes down to a lot of factors, one of which is changes in the shooting culture from the sporting orientation of older generations, to a more self-defense oriented mindset that started with the baby boomers in the 80s and 90s. There was a lot of changes in policing in the 80s and 90s as well, which may have helped the gun prohibition movement to drive divisions there as well.