100% OF AMERICANS SUPPORT 100% No sales to “known or suspected” terrorists Report lost or stolen guns 90% Universal checks for gun buyers Expand mental health treatment Bar sales to mentally ill Bar sales to convicted stalkers Child-proof gun locks Bar sales to all violent criminals Centralized record of gun sales Stronger sentences for illegal guns Require safety training 80% Require gun licenses Require gun safes 3-day waiting period Govt. buybacks of banned guns Honor out-of-state conceal and carry permits Universal checks for ammo buyers Fingerprint gun owners National gun registry 70% National stand-your-ground law School weapons ban Gun purchase limit Assault weapons ban Ammo purchase ban High-capacity magazine ban Guns that microstamp bullets Semi-automatic gun ban Workplace weapons ban 60% 50% OF AMERICANS SUPPORT 50% Demonstrate need for a gun 40% 30% 20% 10% 0% OF AMERICANS SUPPORT 0% 1 2 3 4 5 6 7 8 100% OF AMERICANS SUPPORT 100% Report lost or stolen guns No sales to “known or suspected” terrorists 90% Universal checks for gun buyers Bar sales to mentally ill Child-proof gun locks Bar sales to all violent criminals Stronger sentences for illegal gun Require safety training 80% Require gun licenses Honor out-of-state conceal and carry permits Govt. buybacks of banned guns National gun registry Universal checks for ammo buyers 70% School weapons ban National stand- your-ground law Assault weapons ban High-capacity magazine ban Ammo purchase ban Semi-automatic gun ban Guns that microstamp bullets 60% Workplace weapons ban 50% Demonstrate need for a gun 40% 30% 20% 10% 0% OF AMERICANS SUPPORT 0% 1 2 3 4 5 6 7 8 100% OF AMERICANS SUPPORT 100% Expand mental health treatment Universal checks for gun buyers 90% Child-proof gun locks Require safety training Honor out-of-state conceal and carry permits 80% Require gun licenses National gun registry School weapons ban 70% Assault weapons ban Ammo purchase ban High-capacity magazine ban 60% Workplace weapons ban Guns that microstamp bullets 50% Demonstrate need for a gun 40% 30% 20% 10% 0% OF AMERICANS SUPPORT 0% 1 2 3 4 5 6 7 8

An earlier version of this article ran in January. The mass shooting in Las Vegas on Sunday night has revived an issue that has divided Americans for decades: What can be done do stop such massacres?

President Trump has promised to roll back gun restrictions. The most prominent gun measure currently before Congress is one that would allow people with concealed-weapon permits from one state to carry their weapons to other states.

The concealed carry legislation is one of many gun measures that policy makers have debated in recent years. Backers of those ideas, whether the emphasis is gun rights or gun control, often say the intent is to make Americans safer. We've wondered whether the various ideas politicians talk about would work, and whether the public would support them. In June 2016, we asked Morning Consult, a media and polling firm, to survey two groups: some of the country’s leading experts on gun violence, and a representative sample of the American electorate.

Our expert survey asked dozens of social scientists, lawyers and public health officials how effective each of 29 policies would be in reducing firearm homicide deaths, regardless of their political feasibility or cost. Policies deemed both effective and popular appear in the upper-right corner of the matrix. Less popular, less effective measures fall lower down and to the left.

The two policies ranked most effective were those requiring all sellers to run background checks on anyone who buys a gun, and barring gun sales to people convicted of violent misdemeanors, including domestic assaults. The experts were more skeptical of other much-debated proposals, including a national gun registry and an assault weapons ban. The idea of requiring states to honor out-of-state concealed weapon permits was ranked low.

The academics in our panel — many of the country’s best empirical researchers on gun policy — were far more likely than the general public to support gun control. But nearly all of the policies that experts think could work have widespread support from the general public.

While Americans remain sharply divided in their overall view of the tension between gun control and gun rights, individual proposals are widely favored. The most popular measures in our survey — policies like universal background checks and keeping guns from convicted stalkers — were supported by more than 85 percent of registered voters. Even the least popular idea, a law that would limit gun sales to people who had to demonstrate a “genuine need” for the weapon, was favored by nearly 50 percent.

“We think of guns being an incredibly controversial topic, but what your polling shows and ours has shown is there’s a whole lot of gun policies that really aren’t controversial,” said Daniel Webster, a professor and director of the Johns Hopkins center for gun policy and research.

Public support, of course, doesn’t always translate into legislative action. The Republican Congress, like Mr. Trump, has shown little appetite for measures that would curb gun rights.

What Does Trump Support? 100% OF Americans Support Expand mental health treatment Honor out-of-state conceal and carry permits 0% OF Americans Support 100% of Americans Support Expand mental health treatment Honor out-of-state conceal and carry permits 0% of Americans Support

Historically, Mr. Trump has expressed support for some of the more popular measures on our matrix. But once he entered the presidential race, he came out strongly against changes that might limit gun rights for most Americans. He has said that he supports nationalizing concealed carry permits, and that he wants to lift restrictions on carrying guns in places like schools or military bases.

The concealed carry bill was one of the first introduced in the new Congress. The bill, filed by Richard Hudson, a Republican representative from North Carolina, would require states to honor out-of-state permits to carry a concealed weapon even if the standards for obtaining them differ, just as states honor driver’s licenses from other states.

On our matrix, it is one of the worst-performing ideas. Our panel of experts did not think it would be effective in reducing gun homicides, though a majority of Americans said they’d support it.

Mr Trump has also advocated more treatment for people with mental illness. That idea fared better among the public and the experts in our surveys.

What About Mass Shootings? 100% OF Americans Support Universal checks for gun buyers Expand mental health treatment Bar sales to all violent criminals Assault weapons ban High-capacity magazine ban 0% of Americans Support 100% of Americans Support Universal checks for gun buyers Expand mental health treatment Bar sales to all violent criminals Assault weapons ban High-capacity magazine ban 0% of Americans Support

In addition to asking experts about all firearm homicides, we asked them to rate the same set of policies according to their effectiveness in reducing mass shootings, which make up around 1 percent of gun homicide deaths. Many of their favored policies stayed the same, but a few changed.

Bans on assault weapons and large-capacity ammunition magazines, or limitations on ammunition purchases, would have a greater effect on mass shootings than on routine gun violence, according to the survey. Those rules wouldn’t necessarily reduce the number of mass shootings, the experts said, but could lower the death toll when they occur.

Measures Supported by Academics Opposed to Gun Control 100% OF Americans Support Expand mental health treatment Bar sales to convicted stalkers Bar sales to all violent criminals Honor out-of-state conceal and carry permits National stand- your-ground law 0% OF Americans Support 100% OF Americans Support Bar sales to convicted stalkers Expand mental health treatment Bar sales to all violent criminals Honor out-of-state conceal and carry permits National stand- your-ground law 0% of Americans Support

Most of our experts generally favor gun control policies. Only five said they oppose them, and these five rated nearly every policy option as less effective than their colleagues did. But there were still some policies they thought would reduce gun homicides, including expanded screening and treatment of mental illness, and restrictions on gun sales to people convicted of stalking.

Our gun experts who were opposed to gun control tended to particularly oppose blanket policies. “The essence of a ban is it applies to everyone equally, at least theoretically,” said Gary Kleck, an emeritus professor of criminology and criminal justice at Florida State University. “But in practice, criminals, being criminals, don’t obey the law.”

We also asked the National Rifle Association for its favored gun control policies and its thoughts on our results, but the group declined to comment.

Things Law Enforcement Likes 100% OF Americans Support Bar sales to mentally ill Stronger sentences for illegal gun Honor out-of-state conceal and carry permits Expand mental health treatment 0% OF Americans Support 100% OF Americans Support Bar sales to mentally ill Stronger sentences for illegal guns Honor out-of-state conceal and carry permits Expand mental health treatment 0% OF Americans Support

We asked the membership of two groups of law enforcement professionals to answer our survey (their responses were not combined with those of the academic experts). Just under 100 police officers and chiefs responded, representing only a small percentage of the law enforcement community.

In general, the participating law enforcement professionals expressed more support than the academics for measures that would encourage the use of guns by civilians to defend themselves against crime. They gave higher ratings on stand-your-ground laws and the concealed carry measure, for example. They were far more wary than the academics of measures that involved banning categories of firearms or restricting where individuals can carry guns.

Our sample mixes the two groups of law enforcement professionals, which may obscure differences among them. Previous research has shown that police chiefs tend to be more supportive of policies that restrict gun access, while rank-and-file police officers tend to favor more gun rights.

Other Ideas

In interviews, we asked some of our experts for safety ideas that weren’t included in our survey.

A few pointed to strategies that wouldn’t involve new legislation or regulation, particularly for gang-related gun homicides, which are far more common than mass shootings. Studies have shown that a few programs that focus on community outreach can reduce gun violence in cities. Expanding such programs wouldn’t require new laws, just more funding.

There were also some policies that we didn’t ask about. We didn’t ask about a ban on all handguns, which Washington, D.C., and Chicago have tried. Nor did we ask about barring gun sales to people with a history of alcohol-related offenses, like D.U.I.s. We didn’t ask about lifting restrictions on carrying guns on military bases or near schools, which Mr. Trump has endorsed. And we didn’t ask about a measure, recently passed in a few states, that would allow law enforcement officials or family members to seek a kind of restraining order from a judge to temporarily confiscate guns from a person who appears to be in crisis.