Republicans in Congress have almost universally shut out legislation on gun control in the wake of the Las Vegas massacre, save one idea: banning a tool you can buy online for $200 that makes your semiautomatic weapon fire more like an automatic weapon. One pull of the trigger can spray 30 rounds in a couple of seconds.

The tool is called a “bump fire stock,” or bump stock for short. In 2013, Sen. Dianne Feinstein (D-Calif.) proposed banning them as part of a broader bill to ban assault weapons. Now, with investigators finding at least a dozen bump stocks in the Las Vegas shooter's hotel room, Feinstein and 26 other Democratic senators introduced a bill Wednesday to ban it.

"This is taking it to war," she said. " ... Are these things we want sold over the Internet?"

Some conservative Republicans could get behind that. “You have to have a special class of license to have an automatic weapon,” Rep. Mark Meadows (R-N.C.), chairman of the Freedom Caucus, told the Post. “And, so, if this is something to bypass this, I think it becomes something that we obviously need to look at in the future.”

The very fact that there could be an opening for Congress to do something bipartisan on one of the most partisan issues of our time — gun control — is notable. It also means everyone will soon have an opinion on bump stocks. So here's your primer on what they are and how Congress could regulate them.

Police found 12 weapons with bump-fire stocks in the shooter’s hotel room. These devices can be used to make semi-automatic weapons perform like machine guns. (Patrick Martin/The Washington Post)

What do they do again?: A bump stock is an attachment you can put on your rifle to make it fire more like a machine gun. Semiautomatic guns require you to pull the trigger every time you want to fire a round. Machine guns require just one pull of the trigger to fire dozens of rounds. They have been basically outlawed for decades, with the exception of about 200,000 grandfathered-in guns permitted to civilians with specific licenses. With the bump stock attachment, you can fire hundreds of rounds per minute.

“It's almost like trick shooting,” said David Chipman, a former agent with the Bureau of Alcohol, Tobacco, Firearms and Explosives and an adviser to the gun-control group Americans for Responsible Solutions. “It's not fully automatic, but it's quicker than you could do with your finger alone.”

Is it legal? Yup. The predominant seller, Slide Fire, offers them online. A 2010 letter on Slide Fire's website from ATF indicates that the company petitioned ATF to approve the device as a way to help people with disabilities simulate an automatic weapon without having to hold it a certain way. It's intended to “assist persons whose hands have limited mobility to 'bump-fire' an AR-15 type rifle,” ATF letter says.

Why do some gun owners like it? Well, Slide Fire's website doesn't mention much in the way of, say, testimony from paralyzed veterans who were able to get back into their job thanks to this attachment. The website does, however, have a remarkable marketing pitch that basically boils down to: To own one of these is to be a patriot.

“As long as patriots like you kindle its flame,” a deep-voiced narrator says in a video on the website's homepage as a man puts a bump stock on his rifle and starts firing into the woods, “freedom has but one enemy it cannot defeat. And that is negligence.”

Epic classical music crescendos. The man stops firing, and the gun smokes in the night. The narrator goes on. “Jefferson and Paine, Adams, Madison, Mason and Franklin. I think they're looking down right now at us. I think they understand what we're trying to do.”

The company did not return a phone call for comment Wednesday morning.

Some gun owners don't like it: On the YouTube channel Legally Armed America, a gun owner disses the product as something “only a total jackass would use.” It's not accurate and doesn't help make your gun better, in the sense that you — the holder of it — are safer, he said.

What could Congress do? The bill Feinstein introduced Wednesday will be one of the first efforts to review banning bump stocks individually instead of as part of a larger gun package.

She says she's reaching out to Republican senators to see if they'll support the bill. Some notable Republicans are at least interested in what she has to say.

Rep. Tom Cole (R-Okla.), who is close to House leadership, told MSNBC: “I did not know that there was technology capable that cheaply of transforming a semiautomatic into an automatic weapon. So, yes, I don't think there's any question we ought to look at that.” Later, he told The Takeaway he supports a ban.

Chipman said he'd caution lawmakers against considering an outright ban.

“I found that when you ban something, then everyone wants it,” he said.

Bump fire stocks, Chipman said, are one piece of a larger problem: How everyday people can get military-style assault weapons that not even law enforcement uses regularly.

“The real issue is: 'Can you fire 30 rounds and be able to keep going?'”

But tightly regulating them, the way machine guns are regulated, could be an effective way to limit the bad guys from getting them, he said. Stephen Paddock, the Las Vegas shooter, couldn't buy machine guns to commit mass murder. He had to jerry-rig semiautomatic rifles instead. “That is proof that getting a machine gun is hard,” Chipman said.

If there was ever a moment to consider regulating or even banning this tool, now would be it. It's virtually unheard of in gun crimes to find a perpetrator outfitting a dozen guns with bump stocks.

“It's a worst-case scenario,” Chipman said. “Hotel room. Corner suite. These types of firearms. Cameras. Everything is a '10' on the scale of worst.”