A new lawsuit against the manufacturers of guns used in the 2017 Las Vegas mass shooting claims that AR-15-style rifles are illegal because they are compatible with bump stocks, which increase their rate of fire. The plaintiffs, parents of a woman who was murdered in the Las Vegas massacre, argue that bump stocks like the ones used in that attack convert semi-automatic rifles into illegal machine guns—a position that has been endorsed by the Trump administration. Therefore, they argue, AR-15s are themselves illegal, since the federal definition of machine guns includes firearms that "can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger."

That claim is important, since the 2005 Protection of Lawful Commerce in Arms Act, which generally shields gun makers from liability for crimes committed with their products, includes an exception for "an action in which a manufacturer or seller of a qualified product knowingly violated a State or Federal statute applicable to the sale or marketing of the product." And while the complaint (for reasons that will become clear) does not mention the Trump administration's extralegal administrative ban on bump stocks, the logic of that policy reinforces the plaintiffs' central argument.

Since 1986 federal law has banned the production and sale of new machine guns, including weapons that can be readily converted into machine guns and parts used for that purpose, for civilian use. During the Obama administration, the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) concluded on several occasions that bump stocks, which facilitate a firing technique in which the rifle moves back and forth, repeatedly resetting the trigger and pushing it against the shooter's stationary finger, do not turn rifles into machine guns.

The reason is clear. A rifle equipped with a bump stock does not automatically fire more than one shot for each function of the trigger. It fires one round each time the trigger is activated, and the process is not automatic, since the shooter has to maintain forward pressure on the weapon and keep his finger in position.

Notwithstanding that reality, Donald Trump, in response to the Las Vegas massacre, decided he could ban bump stocks by administrative fiat—the approach favored by the National Rifle Association. He instructed the Justice Department, which includes the ATF, to come up with a rationale, which required defining "function of the trigger" as "pull of the trigger," defining a trigger pull so as to exclude what happens during bump firing, and treating the shooter as part of the rifle mechanism.

The Nevada lawsuit, which was filed in the District Court of Clark County, omits that part of the story, since the fact that the ATF repeatedly said bump stocks were legal before reversing itself under Trump's orders after the Las Vegas attack weakens the case that manufacturers knew before then that they were breaking the law by producing guns that could be equipped with bump stocks. Instead the plaintiffs argue that manufacturers were aware that their guns were compatible with bump stocks and should have recognized that meant they could be readily converted into machine guns, even though the ATF had concluded otherwise.

In light of the bureaucratic history that the lawsuit glides over, that argument is highly dubious. But going forward, if you accept the Trump administration's view of bump stocks, it is at least superficially plausible to maintain that AR-15s are now illegal, since they can be readily equipped with such accessories. In an interview with The New York Times, Lawrence Keane, general counsel for the gun industry's National Shooting Sports Foundation, noted that an AR-15 is "customizable, but the underlying semiautomatic action is not altered." That's true, but it does not matter under the Trump administration's unilateral redefinition of machine guns.

Indeed, the logic of Trump's ban suggests that any semi-automatic rifle is now illegal, since bump firing does not require bump stocks. The ATF concedes that "individuals wishing to replicate the effects of bump-stock-type devices could also use rubber bands, belt loops, or otherwise train their trigger finger to fire more rapidly." The implication is that any semi-automatic rifle, if owned by someone who also has rubber bands, belt loops, or fingers, could be considered a machine gun.

None of that means this lawsuit will succeed. In addition to the problem of proving that manufacturers knowingly violated the law, based on an interpretation that was explicitly rejected by the federal agency charged with enforcing that law, there is the unexamined assumption that the Las Vegas attack would not have happened, or at least would have been less lethal, had the defendants not sold weapons that were compatible with bump stocks.

"The events of October 1 would not have occurred but for the Defendants' illegal

and wrongful conduct," the lawsuit claims. That proposition seems hard to defend, since this was the first time a mass shooter had used bump stocks, and such killers typically use handguns, not AR-15-style rifles. For example, the perpetrator of the 1991 Killeen massacre, which killed 23 people, used pistols, as did the perpetrators of the 2007 Virginia Tech shooting, which killed 32 people, and the Virginia Beach attack that killed 12 people last May.

It's true that the death toll in Las Vegas, which was the deadliest mass shooting in recent U.S. history, was considerably higher, and people tend to assume that bump stocks—as opposed to, say, firing from a height onto a crowd with multiple weapons and lots of ammunition—explain that horrifying achievement. "It was only a question of when—not if—a gunman would take advantage of the ease of modifying AR-15s to fire automatically [sic] in order to substantially increase the body count during a mass shooting," the lawsuit says. But given the well-known tradeoff between speed and accuracy that bump firing entails, it's possible the perpetrator could have killed as many people without bump stocks.

According to the lawsuit, the Las Vegas shooter fired 1,049 rounds in 10 minutes, killing 58 people, or one for every 18 rounds. In the 2018 Parkland shooting, by contrast, the perpetrator fired about 150 rounds in six minutes, killing 17 people, or one for every nine rounds. By that measure, the Las Vegas shooter was half as accurate as the Parkland shooter. Yet the plaintiffs in the Nevada case are implicitly arguing that their daughter would not have died if the Las Vegas shooter had taken the time to aim.

Rep. Thomas Massie (R-Ky.), who leads the Congressional Second Amendment Caucus, argued shortly after the Las Vegas massacre that bump stocks did not necessarily make the attack more deadly. "If he had been more clear-thinking and methodical about it," Massie said, "he could have inflicted more casualties by shooting his firearms without bump stocks." Assuming the Nevada case goes to trial, that issue will be tested in court, along with the claim that gun manufacturers violated a ban that did not exist until Trump decided to rewrite the law.