Congress decided decades ago that automatic weapons should be regulated much more strictly than semi-automatics. Gun rights advocates have accepted these regulations because automatic fire is, by nature, indiscriminate — more like a bomb than a rifle.

For the same reasons, hardware designed effectively to convert legal weapons into machine guns shouldn't be easily available.

From start to finish, buying a machine gun legally takes at least a year, tens of thousands of dollars, and an invasive background check requiring photographs, fingerprints, and lifelong registration with the federal government. Going fully automatic with a "bump stock" takes less than 20 minutes of assembly and just a couple hundred bucks.

According to the Bureau of Alcohol, Tobacco, Firearms, and Explosives, the Las Vegas shooter opted for the second method. He rigged a dozen semi-automatic rifles with bump stocks so they'd fire like machine guns, murdered 59 people, wounded another 500, and left the nation once again reeling.

The usual calls for gun control popped up, and many of them were as impertinent or as irrelevant as usual, not to mention as disdainful of constitutional rights. But the new, relevant, and less intrusive idea of regulating bump stocks has arisen and it is worth supporting.

It would address a legitimate danger without infringing the rights of law-abiding gun owners. Rifles equipped with these devices have the same indiscriminate character as machine guns. They are specifically designed to nullify existing legal restrictions, a hack to get around prudent restraints.

A bump stock works by using the gun's recoil to "bump" the trigger back and forth on a shooter's finger. Because it's a gun accessory, the Obama administration ruled in 2010 that it wouldn't be "regulated as a firearm under the Gun Control Act of the National Firearms Act."

"There isn't going to be any way to get around what this thing does," the editors of the Guns America Blog wrote back in 2011 while reviewing the bump stock. The product, they concluded, "simulated automatic fire."

These poor man's machine gun kits range from roughly $100 to $300 in price. No gunsmith is necessary and no background is check required.

To uphold law and order, legislators must close this gaping loophole. Sen. Dianne Feinstein, D-Calif., has introduced a bill making the devices illegal. Already 20 Senate Democrats have signed as co-sponsors while Republicans from the White House down are considering such a ban carefully. Now they have an opportunity to achieve the first bipartisan gun control bill since Reagan's Firearm Owners Protection Act in 1986.

The National Rifle Association hasn't endorsed legislation, but it has announced that it supports more regulation of bump stocks. As the NRA put it, "devices designed to allow semi-automatic rifles to function like fully automatic rifles should be subject to additional regulations."

It's still not clear what complications there might be to regulating or banning bump stocks. Maybe workarounds, such as three-dimensional printing, which enthusiasts are already discussing, would make any such efforts futile. It's also possible that gun-grabbers will exploit regulatory efforts to open an all-out-assault on gun rights. But as of now, the wisest course of action is neither imploring the ATF to expand its regulatory powers on its own, nor to pass a new legislative ban. Instead, Congress should thoughtfully devise and approve a regulatory system that will bring simulated automatic fire under the regulatory umbrella that covers automatic weapons.

Legalistic or technical distinctions between a machine gun and an AR-15 outfitted with a bump stock are irrelevant. What matters is the result, and the practical effect is that bump stocks make legal rifles into illicit machine guns. Regulating these devices is a move to uphold the law rather than allow it effectively to be flouted. To uphold the law and to prevent another senseless massacre, Congress should regulate them to make them as difficult to come by as real machine guns.