Barely had the massacres in El Paso and Dayton ended than the clamor began for the government to “Do Something” about weapons used in mass shootings.

Once again there were impassioned calls for “common sense” gun control, above all for more sweeping background checks before guns are purchased. “Background checks,” declared Michael Bloomberg, the former New York mayor, are “the two most important words in this debate.” Leading Democrats, including Senators Sherrod Brown, Chuck Schumer, and Bernie Sanders, demanded that Congress pass a law mandating a “universal” background check on all gun purchases. So did President Trump, tweeting that both parties must “come together and get strong background checks.”

In reality, the overwhelming majority of gun sales already require a background check. Anyone who buys a gun from a licensed dealer — whether in person or online, in a store or at a gun show — must be cleared by the FBI before the weapon is delivered. Every year the federal government conducts more than 25 million such background checks — more than 320 million since the system was put in place. The only time the requirement doesn’t apply is when someone acquires a gun locally from a private individual, such as a friend or relative. That’s the so-called “gun show loophole,” which has nothing to do with gun shows and isn’t a loophole, since it doesn’t apply to anyone in the business of selling guns.

Enacting “universal” background checks would mean forcing private citizens, people who aren’t gun dealers, to go through the FBI before they can sell a gun to their next-door neighbor or their sister-in-law. That would impose a considerable burden on the personal affairs of private individuals. But would it “do something” about mass shootings?

This isn’t a new question, and the answer shouldn’t be in doubt. Yet somehow it remains a mystery to a lot of people, even those concerned with public affairs.

In December 2015, two terrorists carrying AR-15 rifles and semiautomatic pistols murdered 14 victims and wounded 22 others in a mass shooting at the Inland Regional Center in San Bernardino, Calif. The next day, the US Senate voted on a bill to expand the federal background gun check system to cover private sales. One of the senators vosting no was Florida’s Marco Rubio.

In an interview the next morning on CBS, he explained that such a law wouldn’t have prevented the latest mass shooting. Neither of the San Bernardino killers was on any database; their background checks had come up clean. When co-host Gayle King asked him about the “many other cases” where such a law could have prevented a massacre, Rubio politely replied that such cases don’t exist.

In “none of the major shootings that have occurred in this country over the last few months or years that have outraged us,” he said, “would [new] gun laws have prevented them.”

A Washington Post staffer urged the paper’s Fact Checker to scrutinize Rubio’s claim, “suggesting that it was almost certainly incorrect.” So the Fact Checker pored through “reams of data,” examining every mass shooting since the Sandy Hook school slaughter in Newtown, Conn. Its conclusion: Rubio was exactly correct. No proposed new law could have prevented those massacres. (In two cases, the background check didn’t work because of a clerical failure.)

Mass shootings are not caused because Congress hasn’t passed “universal” background checks. In almost every instance, the killers buy their guns legally. “Would stronger background checks have stopped El Paso and Dayton?” asked CNN on Monday. Based on everything known so far, no.

Everyone is horrified when a gunman goes on a rampage and turns a school, a church, a nightclub, or a music festival into a bloodbath. Everyone wants to “do something” to make it stop. But guns are already among the most intrusively regulated products in American life, and only a vanishingly tiny fraction of the firearms owned in the United States is ever used to commit a crime. Mass shootings themselves account for only a minuscule fraction of US homicides — and gun violence in America is much less common than it was 25 years ago.

If there were a “common sense” gun regulation that could unfailingly foil mass shootings, we would have adopted it long ago. There isn’t. We are morally bound to try and prevent such carnage, but common sense — and history — say more gun control won’t do it.