Americans who know nothing else about firearms are all too familiar with the name AR-15. It’s the semi-automatic weapon that murderers have used in many of the most notorious and highest-casualty gun killings of recent years: Aurora, Colorado. Newtown, Connecticut. Orlando, Florida. San Bernardino, California.  Now, with modified versions, in Las Vegas, Nevada, and Sutherland Springs, Texas.

What is this gun? Why is it the weapon that people who want to kill a lot of other people, in a hurry, mainly choose? Tim Dickinson offered a useful history of the AR-15’s emergence as the main implement of mass murder last year in Rolling Stone (“All-American Killer: How the AR-15 Became Mass Shooters’ Weapon of Choice”), and Meghan O’Dea in Fortune and Aaron Smith for CNN also had valuable reports.

But there’s another angle of the AR-15 saga that has slightly slipped from view. It is why this particular weapon is so unusually effective in killing things—even when compared with other firearms.

As it happens, I did an Atlantic article on exactly this subject, back in a very different era of American politics. In 1981, I published a book called National Defense, which was popular at the time and was excerpted in three installments in the magazine. One of the installments was called “The M-16: A Bureaucratic Horror Story,” and it included the origin story of the AR-15. That article was not previously available online, but my colleague Annika Neklason has just digitized it from the archives, and it’s now available.

* * *

The AR-15 was newsworthy in those days mainly as the original civilian version of what became the U.S. military’s standard M-16 combat rifle. The problem with the M-16, from the perspective of many of the Americans who had been using it during the 1960s and 1970s in Vietnam, is that it too often failed at the fundamental task of combat weaponry: killing troops on the other side.

The M-16 jammed. It was touchy if it got wet or dirty—which, in jungle warfare, weapons generally did. Veterans’ stories about the M-16, which became newspaper exposés, which became congressional hearings, concerned the battles in which an American soldier or Marine was found shot to death by an enemy AK-47, a jammed M-16 clutched in the American’s hands.

The point of my story was to explain how the Army’s procurement bureaucracy had systematically, and knowingly if not intentionally, converted the early-model AR-15 into the fully “militarized” but vastly less reliable M-16. Those were the comparisons that mattered most in the aftermath of Vietnam: the M-16 versus its AR-15 predecessor, and the M-16 against the adversary’s practically indestructible AK-47.

Along the way, I examined the other side of the comparison: why the AR-15 was such a revolution in killing power. That’s the part of the story that is most relevant now.

* * *

The AR-15, created by the celebrated armaments designer Eugene Stoner, had many advantages, but a crucial one was that it used smaller, lighter bullets than some predecessors, which traveled at higher speed. My story said:

Nearly a century before American troops were ordered into Vietnam, weapons designers had made a discovery in the science of “wound ballistics.” The discovery was that a small, fast-traveling bullet often did a great deal more damage than a larger round when fired into human or (for the experiments) animal flesh. A large artillery round might pass straight through a human body, but a small bullet could act like a gouge. During the early stages of the congressional hearing, [Representative Richard] Ichord asked Eugene Stoner, the designer of the original version of the M-16, to explain the apparent paradox of a small bullet’s destructive power. The answer emerged in the following grisly exchange.

“ICHORD: One army boy told me that he had shot a Vietcong near the eye with an M-14 [which uses a substantially heavier bullet] and the bullet did not make too large a hole on exit, but he shot a Vietcong under similar circumstances in the same place with an M-16 and his whole head was reduced to pulp. This would not appear to make sense. You have greater velocity but the bullet is lighter.”

“STONER: There is the advantage that a small or light bullet has over a heavy one when it comes to wound ballistics. … What it amounts to is the fact that bullets are stabilized to fly through the air, and not through water, or a body, which is approximately the same density as the water. And they are stable as long as they are in the air. When they hit something, they immediately go unstable. … If you are talking about .30-caliber [like a bullet used in the Army’s previous M-14], this might remain stable through a human body. … While a little bullet, being it has a low mass, it senses an instability situation faster and reacts much faster. … this is what makes a little bullet pay off so much in wound ballistics.”

A little bullet pays off so much in wound ballistics. That is what people who choose these weapons know.

I remember one other thing about that story. Everyone I interviewed about these weapons at the time—the AR-15, its bastard offspring M-16, the opposing AK-47—assumed as a first premise that they were talking about battlefield equipment. None of them seemed to imagine such killing power in civilian hands.