Jonathan Blank, left, and Julia Cordover, who survived the Marjory Stoneman Douglas High School mass shooting, listen to President Trump during a meeting on school safety and guns at the White House on Wednesday. (Jonathan Ernst/Reuters)

PRESIDENT TRUMP had very little to say about gun control in his first year in office, even after 58 people were killed on the Las Vegas Strip in the deadliest mass shooting in modern U.S. history last October. But this week — in the aftermath of the school shooting in South Florida that claimed 17 lives and amid a rising student movement for gun control — Mr. Trump signaled he is open to modest gun-control measures, including a ban on “bump stocks” and improved background checks. We would like to believe that Mr. Trump is sincere when he says he has been deeply affected by recent events and wants to bring about change. But his approach to one needed reform — banning the bump stocks that were used to such terrible effect by the Las Vegas gunman — raises questions about just how serious he is.

Mr. Trump said Tuesday he has ordered the Justice Department to issue regulations that would prevent the devices that can be attached to legal semiautomatic weapons to simulate the rapid fire of illegal machine guns. “We cannot merely take actions that make us feel like we are making a difference,” said Mr. Trump. “We must actually make a difference.”

That approach, though, is guaranteed not to produce immediate action. The agency that regulates firearms, the Bureau of Alcohol, Tobacco, Firearms and Explosives, made clear — first in 2010 and again in 2013 — that it does not have the legal authority to ban bump stocks. An attempt to ban the devices through regulation would prompt a battle that would likely tie the issue up in court for years and allow continued sales of bump stocks. No doubt that’s the aim of the National Rifle Association, which coyly suggested it supported a ban even as it opposed legislation to bring it about.

If Mr. Trump is genuinely committed to banning bump stocks, he should back legislation sponsored by Sen. Dianne Feinstein (D-Calif.) that would bar them, along with other devices that have been created to circumvent the intent of federal gun laws. And he should recognize that eliminating bump stocks falls far short of a solution to the epidemic of mass shootings. Semiautomatic, assault-style weapons — like the one used without a bump stock at Florida’s Marjory Stoneman Douglas High School — continue to pose a risk to public safety and should be banned or, at the very least, subject to stringent regulation. That a 19-year-old with a history of troubled behavior was able to easily and legally acquire an AR-15 rifle underscores the need for better controls.

Mr. Trump also signaled interest in legislation that would strengthen the system of national background checks as well as raise the minimum age for purchase of certain weapons. Congress should take his cue and pursue measures, including pending legislation on background checks, that would enact these small reforms. Though not an adequate response to the students, it would be a start.