Today the Justice Department finalized its ban on bump stocks, which Acting Attorney General Matthew Whitaker claims merely "clarifies" federal law. It actually rewrites federal law, a function the Constitution assigns to Congress. Whitaker also wants us to believe that the bump stock ban shows "President Donald Trump is a law and order president." To the contrary, it shows he is a president who ignores the law whenever it proves inconvenient.

The final rule defines "bump-stock-type devices" as "machineguns" under the National Firearms Act of 1934 and the Gun Control Act of 1968 "because such devices allow a shooter of a semiautomatic firearm to initiate a continuous firing cycle with a single pull of the trigger." That understanding of the law contradicts the plain language of the NFA, the position repeatedly taken by the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) during the Obama administration, and the interpretation endorsed by both supporters and opponents of a legislative ban.

The NFA defines a machine gun as "any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger." A bump stock harnesses recoil energy to make a rifle slide back after it is fired, resetting the trigger. If the shooter maintains forward pressure on the barrel, the trigger will bump against his stationary finger, causing the gun to fire again, and so on. This technique increases the rate of fire, but the rifle is still shooting just one round for each function of the trigger.

The Justice Department had to find a way around the clear meaning of the law because the president promised to ban bump stocks by administrative fiat after they were used by the perpetrator of an attack that killed 58 people in Las Vegas last year. The DOJ accomplished that trick by defining "a single function of the trigger" as "a single pull of the trigger" and defining pull to exclude what happens during bump fire. According to this account, when the trigger is activated by bumping against the trigger finger, that is not, contrary to logic and appearances, a "function of the trigger."

Another problem for DOJ was that a rifle equipped with a bump stock does not operate "automatically," since the shooter has to maintain "constant forward pressure with the non-trigger hand on the barrel-shroud or fore-grip of the rifle, and constant rearward pressure on the device's extension ledge with the shooter's trigger finger," as the rule notes. The Justice Department resolved that difficulty by treating the shooter as part of the rifle mechanism.

According to the rule, "these devices convert an otherwise semiautomatic firearm into a machinegun by functioning as a self-acting or self-regulating mechanism that harnesses the recoil energy of the semiautomatic firearm in a manner that allows the trigger to reset and continue firing without additional physical manipulation of the trigger by the shooter." That gloss is accurate only if you ignore the role of the human who pushes the rifle forward to engage the trigger and if you insist that activating the trigger in that manner does not count as "physical manipulation of the trigger." The DOJ claims this counterintuitive perspective reflects "the best interpretation of the term 'machinegun,'" by which it means the interpretation that facilitates the result demanded by the president.

As Joshua Prince and Adam Kraut, lawyers representing the Firearms Policy Coalition, noted in comments on the rule, the DOJ's "interpretative jiggery-pokery" is "pure applesauce." It not only is inconsistent with what everyone previously understood the law to mean, but it arbitrarily targets certain products when the DOJ's reasoning would cover all manner of jury-rigged setups that make bump fire possible. "An individual does not require a bump-stock device in order to bump-fire a factory semi-automatic firearm," Prince and Kraut write. "ATF readily acknowledges that bump-firing can be lawfully achieved through the 'use [of] rubber bands, belt loops, or [to] otherwise train their trigger finger to fire more rapidly,' in a clear statement of its intent to unequally apply the law."

Given those alternatives, not to mention the tradeoff between speed and accuracy for shooters using bump stocks, it is doubtful that banning them will have any noticeable impact on the lethality of mass shootings. At the same time, it seems like they ought to be covered by the Second Amendment as arms that are "in common use for lawful purposes," even if they are by no means necessary to exercise the constitutional right to armed self-defense. But the question is not whether banning bump stocks is a good idea. The question is whether bump stocks are already banned.

Although the Obama administration was much more supportive of gun control than the Trump administration, it repeatedly declared that bump stocks were legal, meaning that banning them would require a new act of Congress. Sen. Dianne Feinstein (D-Calif.), a dogged gun controller, agreed. This is one of the few gun-related questions on which Feinstein sees eye to eye with Rep. Thomas Massie (R-Ky.), leader of the Congressional Second Amendment Caucus. When you look at the law, you can see why: Only by stretching and distorting it can you achieve the end ordered by Trump.

Politically, a bump stock ban makes sense. Like most gun control measures, it creates the appearance of doing something about deadly violence. Furthermore, this particular measure won't raise the hackles of the National Rifle Association, which pre-emptively endorsed an administrative ban on bump stocks, or open the door to more ambitious restrictions, as seeking new legislation from Congress might. Next to those benefits, the political cost of turning otherwise law-abiding gun tinkerers into felons if they fail to destroy or surrender their property seems negligible, notwithstanding the opposition of a few smaller gun rights groups.

Legally, however, the precedent set by Trump's bump stock ban is troubling, regardless of how you feel about the Second Amendment or define its scope. The president does not have the authority to rewrite laws that interfere with his agenda, whether the subject is guns or immigration. Principled critics of this administration should call Trump out whenever he oversteps his legal powers, even when they happen to like the outcome.

Update: Today the Firearms Policy Coalition filed a federal lawsuit challenging Trump's bump stock ban. "The ATF has misled the public about bump-stock devices," said Joshua Prince, one of the group's lawyers. "Worse, they are actively attempting to make felons out of people who relied on their legal opinions to lawfully acquire and possess devices the government unilaterally, unconstitutionally, and improperly decided to reclassify as 'machineguns.'"