How do mass shooters choose their weapons?

It is no coincidence that many mass killings have been committed using the same type of weapon. Contrary to popular belief, however, the military-style semiautomatic rifles perceived to be the preferred weapons of choice among mass shooters are not the most lethal firearms available.

“We’re all trying to understand something that is not within our capacity to comprehend,” said Mike Bazinet, spokesman for the National Shooting Sports Foundation trade association. “I can’t get in the mind of a killer. … They don’t seem to be choosing [weapons] that would cause the most damage. For the most part they’re using the most popular guns in America.”

In recent years semi-automatic rifles were used in mass-shootings at the Tree of Life synagogue in Pittsburgh; a Waffle House in Nashville, Tenn.; a nonprofit public benefit corporation in San Bernardino, Calif.; schools in Parkland, Fla., and Newtown., Conn.; a nightclub in Orlando, Fla.; a concert in Las Vegas, Nev.; a movie theater in Aurora, Colo.; and a church in Sutherland Springs, Texas.

In documenting mass murder, descriptions and definitions are typically inconsistent, hampering the fair exchange of information. According to the Federal Bureau of Information a “mass murder” or “mass shooting incident” involves four or more victims within one event occurring in one or more locations in close proximity. A semiautomatic rifle fires one shot and reloads with one pull of the trigger. An automatic rifle, or machine gun, is capable of firing multiple rounds with a single trigger pull. An “assault rifle” is capable of fully automatic fire and used in military assaults.

Rifles of any kind are seldom used in homicides. Of 66,231 U.S. murders from 2012 to 2016, the FBI counted 1,473, about 2%, committed with long guns. Rifles were used in 43 mass shootings from 1982 to 2019, and 69% of those involved military-style semiautomatics.

“They’re light, easy to use and easy to carry,” Shira Goodman, executive director of CeaseFire PA, a gun-violence prevention association, said of the military-style semiautomatic rifle. “They’re a civilian version of a military weapon — a lot of men want to have that.”

Commonly called “assault rifles,” “tactical arms” or “black guns,” they do not have the military capability suggested by those monikers. Called “modern sporting rifles” by the firearms industry, they are not a recent development. Erroneously called AR-15s, most are derivatives of the long forgotten original.

In the late 1950s a lightweight combat rifle was released by Armalite Inc., a small arms manufacturer. It was adapted by the U.S. military as the M16, a rifle made specifically for military assaults that easily could be switched from burst mode (three shots with each trigger pull) to semiautomatic to automatic fire.

In 1964 Armalite was purchased by Colt, a Connecticut-based company that remodeled and repackaged the gun as the AR-15 Sporter (AR stands for Armalite Rifle). Colt made 80 design changes, limited the rifle’s firepower to semiautomatic action only and marketed it to hunters. The campaign failed — hunters at the time preferred conventional-style sporting arms over the military look of the ARs.

A 1989 ban restricted the importation of foreign-made semiautomatic rifles said to have no “legitimate sporting use,” and in 1990 just 74,000 AR-style rifles were manufactured by several companies in the United States, according to the National Shooting Sports Federation.

Counterintuitively to some, the sale of military-style assault rifles spiked during the 1994 to 2004 Federal Assault Weapons Ban. The law restricted nearly 20 types of

semiautomatic weapons and copies in any caliber, and banned specific models of assault-style rifles manufactured by Colt, SWD, Intratec and others. It defined semiautomatic rifles as those able to accept detachable magazines and two or more of the following: pistol grip, bayonet mount, folding or telescoping stock, grenade launcher and flash suppressor or a barrel designed to accommodate one.

But riddled with loopholes, the ban backfired. By 2003, the year before it was lifted, an estimated 380,000 AR-derivative rifles were being legally made in America, according to NSSF. A 2004 University of Pennsylvania study commissioned by the National Institute of Justice stated that while some firearm models were explicitly banned, “the inclusion in the list of features that were purely cosmetic in nature created a loophole that allowed manufacturers to successfully circumvent the law by making minor modifications to the weapons they already produced.”

The sale of “modern sporting rifles” spiked again, from 633,000 in 2008 to 1,006,000 in 2009, the year Barack Obama became president. With the ban lifted and expectations among many gun owners that Mr. Obama would strengthen gun restrictions in his second term, AR sales doubled in 2013 to some 2,275,000. From 2000 to 2015, the number of companies manufacturing AR variants rose from 29 to about 500.

The National Shooting Sports Federation estimates that in 2016 there were more than 16 million AR-style rifles in the United States, with prices ranging from less than $500 to several thousand dollars. Semiautomatic weapons of all types now account for 20% to 25% of some 350 million privately owned firearms in the United States. The percentage is rising. Young target shooters and hunters are more accepting of the military style, and semiautomatic rifles account for a growing percentage of the 10 million to 15 million new firearms bought annually.

“For 18 or 19 years, we’re seeing former soldiers coming home from the Middle East who are comfortable with the design and want to use [AR derivatives] for shooting and hunting,” said Mr. Bazinet. “They are growing in popularity among hunters. The ARs now sell far more than conventional-style rifles.”

In 2017 Pennsylvania became the last state to permit semiautomatic rifles for hunting, but only for small game and varmints.

An NRA spokeswoman declined to go on the record but provided background information explaining the popularity of the ARs. Unlike conventional firearms, the military-style modular design enables the gun to be easily customized for specific purposes. Switchable stocks, combs, forestocks, pistol grips, cartridge-carrying magazines, sights and even barrels make the guns more versatile than any other firearm. Since the 1990s, semiautomatic rifles have been the primary firearms used for national and state marksmanship competitions, independent shooting practice and home defense.

It’s interesting to note the legally available rifles that are typically not chosen for use in mass shootings.

■ Some conventional-style semiautomatic rifles fire the same cartridges as the assault-style rifles and can accept 10-, 20- and 30-round magazines similar to those used in AR-derivatives. With polished wood stocks and forestocks, sporting arms manufactured by companies including Ruger and Browning look like grandpa’s deer rifle but shoot like the guns that superficially mimic a military design. Mr. Bazinet noted that those guns have not been used in mass shootings and would not be restricted in state and federal legislation attempting to restrict or ban “military assault weapons.”

■ Rarely do mass murderers use the large-bore highcaliber rifles preferred by big-game hunters for their tremendous .30- or .40-caliber stopping power. The semiautomatics associated with mass murders generally fire small-bore .223 Remington or 5.56 x 45-millimeter NATO ammunition that causes less damage but is designed to accommodate high-precision semiautomatic actions.

■ Fully automatic rifles are designed to inflict maximum carnage in battle. Machine guns manufactured before 1986 can be legally owned by Americans but are heavily regulated, expensive and rarely used in mass killings. According to a 2015 report from the Congressional Research Service, an off-duty police officer committed such a crime using a legally registered machine gun that had been issued to him by his department. The automatic weapon was of a type that may have been included in the expired Federal Assault Weapons Ban.

■ The same congressional report found one mass killing in which a semiautomatic rifle had been illegally converted into a fully automatic weapon. It happened in 2011 when a mentally challenged man entered a Nevada restaurant, killed four people and wounded another seven.

■ Although semiautomatic and automatic rifles can be illegally purchased on the black market, the 2012 to 2016 FBI report found that most rifles used in mass shootings were obtained legally through the standard federal background check.

Without a magazine, semiautomatic rifles fire just one round at a time, pushed manually into the chamber. Some potential victims have escaped mass killings, and shooters were subdued or killed, while empty magazines were being replaced.

“When the guy who shot up a Waffle House in Nashville went to change magazines [on a semiautomatic rifle], an unarmed civilian tackled him. Magazine capacity is a big issue,” said Ms. Goodman. “Hopefully people will come up with new bills that keep these guns away from criminals without taking them away from lawabiding gun users.”