Pressed to take action on guns by students who survived the most recent mass school shooting and by President Trump, lawmakers are discussing what they might enact on gun safety. On Wednesday, the president surprised the bipartisan members of Congress he met with when he sided with Democrats on expanding background checks and raising the purchasing age for rifles.

"It doesn't make sense that I have to wait til I'm 21 to get a handgun but I can get this weapon at 18," Mr. Trump said. He also discouraged Republican hopes of passing a concealed carry law across state lines, and he seemed to support the idea of taking guns from people who show signs of threatening behavior, as the alleged Florida shooter did.

Lawmakers are talking about several different ideas, though it's not completely clear what will attract enough bipartisan support to pass, much less be grouped into a single bill, as Mr. Trump suggested Wednesday. Here's a rundown of some of the proposals:

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The "bump stock" ban

Bump stocks are a device that allow semi-automatic weapons, such as the AR-15, to mimic the firing rate of automatic weapons, which are already the subject of strict regulation. The man who killed 58 people in Las Vegas in October utilized a bump stock, which helped him fire more than a thousand rounds from his perch above a country music concert in the city.

In the wake of that massacre, California Democratic Sen. Dianne Feinstein introduced a bill to ban bump stocks, and there were some early signs that the bill might garner bipartisan support. But since October, the bill has languished in the Senate amid opposition from the NRA to a legislative fix.

Mr. Trump ordered the Justice Department last week to begin work on banning bump stocks through the regulatory process. "I'm writing that out myself," he said about bump stocks this week. "I don't care if Congress does it or not. I'm writing it out myself." On Tuesday, Attorney General Jeff Sessions said that the government could ban bump stocks through the regulatory process without going through Congress. The ATF ruled in 2010 that because bump stocks are neither machine guns nor firearms, they could not be regulated by the ATF. It has said that it's up to Congress to prohibit the devices.

Feinstein, citing the ATF's legal opinions, agrees that it's up to Congress to make a bump stock ban permanent. "If ATF tries to ban these devices after admitting repeatedly that it lacks the authority to do so, that process could be tied up in court for years, and that would mean bump stocks would continue to be sold. Legislation is the only answer," she said in a statement on Tuesday.

Assault Weapons ban

On Monday, Democratic Reps. Ted Deutch and David Cicilline introduced a ban on assault weapons in response to the Parkland shooting. The bill would make it "unlawful for a person to import, sell, manufacture, transfer, or possess, in or affecting interstate or foreign commerce, a semiautomatic assault weapon."

"Assault weapon" is a somewhat nebulous category of firearm, a point gun control opponents often make. In a New York Times op-ed supporting a ban, Republican Rep. Brian Mast wrote that Congress should define "what constitutes an assault or tactical firearm…The exact definition of assault weapon will need to be determined. But we should all be able to agree that the civilian version of the very deadly weapon that the Army issued to me should certainly qualify."

An assault weapons ban was in effect from 1994 to 2004, when it expired. Proponents of a similar ban today say that it was effective and that a new one could lower the number of mass shootings. And they point to the AR-15, which was used in the Parkland shooting, the Sandy Hook massacre in 2012, the Las Vegas massacre, and a number of other high-casualty incidents as a perfect example of a firearm that should not be sold to civilians.

But an assault weapons ban is a non-starter in Congress. The NRA and the White House have already both said they oppose such a ban, and it's hard to imagine many Republicans joining Mast on this front.

The AR-15 is very popular among gun-owning Americans. It's an easy gun to use, quite accurate, reliable, and relatively cheap. And despite its notoriety, opponents of a ban note that rifles are used in crimes far less regularly than handguns. According to the FBI, rifles – a category that includes everything from the AR-15 to more traditional hunting guns -- were used in just 2.4 percent of murders between 2009 and 2014.

While it's hard to say how many AR-15s are owned by Americans, the National Shooting Sports Foundation puts the number at somewhere between 5 and 10 million. The Democratic assault weapons ban wouldn't confiscate these weapons when they are legally owned. Still, the popularity of the AR-15 and guns like it, not to mention the strength of the gun lobby, means that it's hard to imagine a new ban being passed anytime in the near future.

Arming teachers

The proposal President Trump has been pushing often is arming "gun-adept" teachers and other school personnel. It's considered one of the most far-reaching and controversial proposals he's suggested.

At the Conservative Political Action Conference just outside Washington, D.C., last week, Mr. Trump said an armed teacher would have "shot the hell out of" the Florida shooter. Mr. Trump has suggested offering teachers who undergo training and are willing to carry guns bonuses of up to 40 percent of their salaries.

While some of the most conservative Republicans may agree with Mr. Trump, no one has suggested any legislation yet. Democrats have dubbed the proposal a non-starter.

But Mr. Trump has also seemed to indicate such a proposal would be up to the states, as he tweeted Saturday.

Fix NICS

Of all the ideas so floated far, this may have the best chance of passage. The bipartisan "Fix NICs" proposal led by Sen. John Cornyn, R-Texas, and Sen. Chris Murphy, D-Connecticut, aims to improve the communication between local, state and federal law enforcement to strengthen the background check system. Specifically, the bill would punish federal agencies that fail to report criminal offenses to the National Instant Criminal Background Check System, among other things.

But some Democrats are frustrated that the legislation doesn't require universal background checks, something Democrats have long desired. Current federal law does not require unlicensed firearm sellers to run background checks on potential customers.

"What will prevent future tragedy? Comprehensive background checks will. The Fix NICS bill will not," Senate Minority Leader Chuck Schumer, D-New York, sad on the Senate floor Tuesday.

Republicans have tried to pair the Fix NICS proposal with a measure to allow concealed carry across state lines. In a bipartisan meeting with lawmakers Wednesday, Mr. Trump shot down the idea of attaching concealed carry reciprocity to any background check bill, saying it would never pass.

Red flags

"Red flag" laws aim to keep guns out of the hands of people who are a threat either to themselves or others. The National Review recently urged conservatives to consider this proposal, which amounts to a kind of gun violence restraining order (GVRO), and this is an idea the president seemed interested in Wednesday.

How would it work? A GVRO would enable family members or people close to a potential shooter to petition for a temporary order to take an individual's guns away and prevent that person from buying a gun. Petitioners would show a court evidence that the individual is dangerous, but the individual would also be able to respond to the claims. The order would lapse after a period of time unless petitioners successfully argue that the ban should continue.

Florida Sen. Marco Rubio embraced the idea last week, tweeting "Not just me calling for conservatives to consider "red flag" gun law.

Not just me calling for conservatives to consider “red flag” gun law. https://t.co/GIpXe7UujT Read This: https://t.co/TtzrVmu1h1 #Sayfie — Marco Rubio (@marcorubio) February 18, 2018

Rubio plans to introduce a new bill that will lead to the creation of gun violence restraining orders.

The policy is already in effect in California, Oregon, Connecticut, Indiana and Washington. And, in response to the Parkland, Florida shooting, Rhode Island just became the sixth state to adopt a red-flag approach, under an executive order signed by Gov. Gina Raimondo. Law enforcement agencies in Rhode Island will be able to evaluate red flags -- including threats made in person or on social media or videos. And several other states are also considering red-flag laws.

This proposal is a direct response to reports that there were signs prior to the shooting that alleged Florida shooter Nikolas Cruz was a danger. He posted on YouTube in September, "Im going to be a professional school shooter." He had threatened his mother, posted some photos of himself with weapons and other unsettling pictures of eviscerated small animals. There were calls warning law enforcement that people were worried about what Cruz might do.

Some lawmakers are convinced a red flag law could help prevent other mass shootings and think that a measure like this both targets the mental health concerns and would not permanently take guns away from anyone who should be able to carry a gun. Even so, the odds seem stacked against the GVRO, since many Republicans remain unconvinced that it would be an effective deterrent.