Adam Gaffney is an Instructor in medicine at Harvard Medical School and a pulmonary and critical care physician at the Cambridge Health Alliance and serves on the board of directors of Physicians for a National Health Program.

A gun shop in Merrimack, N.H. (Dominick Reuter/AFP)

There comes a moment, soon after an appalling mass shooting, when many horrified people cry out in exasperation: Explain, please, why is it that anybody in America needs an automatic weapon? By this they mean the rapidly firing weapons that are at the center of many of today’s gun control debates.

But the moment they’re brought up, someone inevitably counters: You don’t mean automatic weapons — what you mean to say is semiautomatic! Learn a thing about guns before running your mouth!

Now this contingent (however hairsplitting and off-putting their response may seem) is basically correct: Closely regulated by the National Firearms Act of 1934 (passed in response to fears of Tommy gun-wielding bank robbers), the sale of automatic weapons (or “machine guns”) — which continually spit out bullets as long as the trigger is held down — was all but eliminated by the Firearm Owners Protection Act of 1986. As a result, as noted in Slateautomatic weapons are today very regulated, expensive and rare and their use in crime basically negligible. Moreover, the vast majority of mass shootings are carried out with semiautomatic — not fully automatic — firearms.

Surprisingly, the gut-wrenching, incomprehensible carnage of Sunday’s massacre in Nevada may have been an exception to this: Reports suggest that one way or another — perhaps by the addition of a device called a “bump-stock” — the shooter used some automatic fire. But this is admittedly extremely rare, and mostly beside the point. Had he committed the same crime in the same manner but with a semiautomatic rifle and a tall pile of large-capacity magazines, the carnage could probably have been just as bad.

The reality is this: A semiautomatic firearm — whether a long gun or a handgun — is a weapon of profound destructive capacity. Though semiautomatic (as opposed to automatic) guns require that the trigger be pulled once to discharge each bullet, they still allow a shooter with a quick finger to unleash an avalanche of bullets at a horrifying pace.  When unloaded into a densely packed space, the resultant hail of bullets is just as capable of inflicting massive damage. Once fired, each of these projectiles then cuts a path through body, stretching adjacent tissues and so creating a path of destruction — the “wound cavity” — that can widen even more as the bullet tumbles and sometimes fragments, creating new smaller missiles each with their own trajectory; more bullets thus mean more punctured lungs, more shattered bones, more lacerated arteries, more severed spines.

In short, semiautomatic weapons are capable of inflicting egregious harm on a large number of people in a very short period, even when wielded by deranged users with no remarkable shooting skills. For instance, Adam Lanza, the perpetrator of the Sandy Hook massacre, used a semiautomatic rifle to fire 154 rounds and kill 26 individuals in under five minutes, as MSNBC reported; Seung-Hui Cho, responsible for the Virginia Tech massacre, fired 170 rounds with a pair of semiautomatic pistols in only nine minutes, leaving 30 dead, according to The Washington Post. To the extent that anything should be done about the types of guns available in America, we should end the sale (as others have previously argued and as the American College of Physicians suggested in a statement Monday) of all semiautomatic firearms, whether handgun or long gun.

Police found 12 weapons with bump-fire stocks in the shooter's hotel room. These devices can be used to make semi-automatic weapons perform like machine guns. (Patrick Martin/The Washington Post)

The liberal obsession with “assault weapons” is thus a colossal mistake. The assault weapons ban identified such firearms by a host of largely cosmetic characteristics that were mostly irrelevant to their lethal potential and had no discernible effect on overall gun deaths. Collapsible stocks, pistol grips, bayonet mounts — who cares? Perhaps there is some utility to some of these features in combat settings, but when it comes to a mass shooter in a civilian environment, what matters more than anything is the sheer quantity of terrifying projectiles flying through crowded spaces at thousands of feet per second. To a lesser extent, things like the kinetic energy of a bullet also matter, but this can actually be higher in traditional hunting rifles than so-called assault weapons.

For all the fuss about mistaken references to automatic weapons versus semiautomatic ones, it is not clear that the former are always more lethal, insofar as they are more unwieldy to fire and thus inaccurate.  Here is how a 2008 Army Manual on “Rifle Marksmanship” describes the advantages of semiautomatic over automatic fire, as cited in Rolling Stone: “Automatic or burst fire is inherently less accurate than semiautomatic fire. … The most important firing technique during fast-moving, modern combat is rapid semiautomatic fire.”

What would a ban on the sale of semiautomatic weapons mean for shooters?

It would mean that pistols would be “single-action”: After discharging the gun, the shooter would have to cock the hammer to fire again. Long guns would require the operation of a lever, a pump or a bolt — already typical of many of the guns used in hunting and target shooting currently — after each shot.

Some might invoke constitutional challenges to a semiautomatic weapons ban, which seems irrelevant: If it is entirely constitutional to restrict access to automatic firearms, why not semiautomatics? In any event, the Supreme Court has thus far declined to hear at least one challenge to a ban on semiautomatic rifles.

Others might contend that non-semiautomatic weapons would be inadequate to resist a tyrannical U.S. government. They are entirely right, but that is equally true of semiautomatics. To effectively resist the American government, one would need at least a battalion of tanks, batteries of powerful artillery, an array of antiaircraft missiles, and so on and so forth. In short: nothing you could buy at your local gun shop at any rate.

This kind of a shift in the national weapon stock could have advantages outside the context of mass shootings: The fewer projectiles whizzing through the air during a street corner firefight, the better.

There are no simple solutions to drastically reduce America’s gun violence. Much needs to be done on many fronts, from expanding mental health access to addressing the adverse socioeconomic circumstances that engender street crime and day-to-day violence in the first place. Nevertheless, much will be said about gun control in coming days. Many proposals will be offered, and realistically nothing will be accomplished — at least in the short term. As political circumstances change, however, some good may be able to be done. When that happens, insofar as we want to do anything about the types of guns people can buy, we should aim for an across-the-board semiautomatic firearms ban.