When a gunman armed with an AR-15 or AK-47 murders and injures shoppers, bar patrons, worshipers, or schoolchildren, many Americans reflexively focus on the means of the bloodshed. The spectacle of civilians killed with powerful weapons designed for battlefield use understandably evokes outrage. Critics argue they have no legitimate civilian use, and are too lethal to be legal.

Following this summer’s outbreak of mass shootings, survivors, cultural figures, and national Democrats have called for a new federal assault weapons ban. Representative David Cicilline of Rhode Island has introduced a bill that would create such a policy, and it now has 201 co-sponsors, including one Republican. The demands for prohibition have revived a nearly three-decade-long debate over whether banning particular weapons is good policy.

From 1994 to 2004, the United States had a federal ban on new production of assault-style rifles. The law proved controversial, both for its perceived electoral consequences and disputed effects on crime. State-level assault weapons bans enacted in the years since have been undermined by noncompliant gun owners, easily surmounted design requirements, and legal and political calculations that allow existing military-inspired rifles to remain in circulation. But some data analyses do suggest that the lapsed federal law did accomplish its primary purpose: reducing the frequency and deadliness of gun rampages. Other research shows that a component of the law outlawing high-capacity ammunition magazines may have limited the use of those devices in shootings.

Here is the research that will inform a push for a new federal assault weapons ban, ideas for improving on the prior version, and alternative proposals for limiting the risks that mass shooters pose.

What did the 1994 federal assault weapons ban do?

Guns like the AR-15 and AK-47 are frequently described by pundits as “weapons of war.” But those firearms have become a hallmark of certain corners of American life. More than two decades ago, politicians around the country and in both parties who wanted to reckon with assault weapons’ proliferation started talking about how to define and control the guns in the aftermath of several highly publicized mass shootings.

The 1994 federal law prohibited the manufacture for sale to civilians of semiautomatic rifles with detachable magazines and certain additional features, including pistol grips, bayonet mounts, or folding stocks. The ban also extended to a narrow set of semiautomatic handguns that could accept detachable magazines, as well as the manufacture of all ammunition magazines that could hold 10 rounds or more.

The federal law was not a ban on owning an assault-style gun, however. Weapons and magazines produced before it went into effect were grandfathered in. As journalist Paul Barrett detailed in his book Glock, some companies actually ramped up production of soon-to-be-outlawed components in the lead-up to the law, swelling the supply of products that would be exempt once the ban kicked in. Notably, if you owned an AR-15 or 30-round magazine before than ban, you were still allowed to sell them under the law.

Did the federal ban reduce gun violence?

Perhaps the most comprehensive review of gun laws to date was undertaken in 2018 by the RAND Corporation. On the question of assault weapons bans, RAND found existing studies were either inconclusive or presented only limited evidence of their effectiveness at reducing crime and mass shootings. The strongest impact RAND identified was on the gun industry itself, “including impacts on production, price, and potential spillovers from primary to secondary markets.”

One major reason the federal ban may have had a minimal effect on the overall criminal use of assault weapons is that such firearms were already implicated in only a small share of shootings before the law was enacted. According to a 2004 National Institute of Justice study, assault weapons were only used in 8 percent of violent crimes — at most — in the decade prior to the ban. That figure has not meaningfully changed in the decade and a half since then, with one study finding that assault weapons comprise 2.5 to 8.5 percent of crime guns in selected cities.

Then there was the law’s grandfather clause, which kept thousands of assault-style weapons in circulation. When the Urban Institute evaluated the ban in a 1997 paper, the think tank concluded that the exemption significantly sapped the statute’s crime-stopping power.

Meanwhile, the law’s magazine provisions appeared to have uneven results. The 2004 NIJ study looked at guns equipped with large-capacity magazines recovered by police in Baltimore, Milwaukee, Anchorage, and Louisville. In all but one of those cities, the number of high-capacity magazines at crime scenes continued to grow or merely plateaued.

The exception was Baltimore, where recoveries of high-capacity magazines had fallen by nearly 25 percent as of 2003. Separate data obtained by The Washington Post in 2013 from the Virginia State Police suggests that the magazine ban was very effective in that state: The police force’s recoveries of high-capacity magazines fell from 1,028 in 1994 to just 552 in 2004.

What happened to the overall numbers after the ban expired?

“By the time the ban was lifted in 2004, we were starting to see a decrease in criminal use of assault weapons and high-capacity magazines,” said Christopher Koper, a George Mason University criminologist who was the principal author of the 2004 NIJ study. Rather than an assault weapons and high-capacity magazine ban yielding a quick reduction in shootings, “it might take some time to get the desired effects.”

In the decade and a half since Congress allowed the ban to lapse, criminal use of semiautomatic weapons equipped with large detachable magazines appears to have increased in many places, according to several studies. A 2017 study in the Journal of Urban Health found that in Baltimore, Richmond, and Minneapolis, the prevalence of such firearms relative to all recovered crime guns increased by 48 to 111 percent in the years after the ban expired. Likewise, according to the data analyzed by The Post, Virginia State Police recoveries of large magazines rose back above 1,000 by 2007.

Did the federal ban reduce the severity of mass shootings in particular?

When looking more narrowly at active shooter incidents, researchers have found a correlation between the law and less frequent, less deadly gun rampages. There’s a logic to that finding: The weapons the ban covered are disproportionately used in mass shootings, so there was greater potential for them to make a difference with those crimes.

Louis Klarevas is a research professor at Columbia University Teachers College and the author of Rampage Nation, which examines American mass shootings and efforts to prevent the killings. His research looked at mass shootings resulting in six or more deaths, which he refers to as massacres, because they are “the most dangerous and threatening to American public safety.”

He found that deaths in such high-fatality mass shootings dropped by 25 percent under the ban. Massacre deaths involving assault weapons fell by 40 percent, fatalities involving both assault weapons and high-capacity magazines fell by 54 percent. After the ban lapsed, mass shootings and related deaths surged. “We know that during the federal assault weapon ban, we saw some significant reductions in high-fatality mass shootings compared to the decade before the ban and the decade after it expired,” he said. “What we don’t know exactly is the mechanism behind that change.”

A 2019 study looking at the impact of the federal law on mass shooting deaths arrived at a similar conclusion: While new assault weapons and high-capacity magazines were outlawed, mass shooting fatalities were 70 percent less likely than during the decades before and since.

What about state assault weapon laws?

Seven states and the District of Columbia have their own assault weapons bans on the books today. In some cases, the state laws go further than the federal law did. California, Connecticut, Hawaii, New Jersey, and New York require assault weapons legally owned prior to the imposition of the bans to be registered. California, Connecticut, Hawaii, Maryland, and New York also bar owners of pre-ban weapons from transferring firearms to anyone else. California also has a law prohibiting the possession of high-capacity magazines, with no grandfather clause. In April, a federal judge declared the magazine ban unconstitutional, though manufacture and sale of the devices remains prohibited, pending an appeal.

What does the data say about the effectiveness of state assault weapons bans?

Even less than what’s known about the 10-year federal ban. There’s simply not much research out there.

Mark Gius of Quinnipiac University is one of the small number of academics who has examined state-level assault weapon bans. He found that they did not reduce gun homicides in general. However, his research found that state laws were associated with fewer mass shooting deaths.

Given the patchwork nature of state gun laws, it can be easier to evade them than a federal ban, as was demonstrated in the recent Gilroy, California, Garlic Festival shooting. Though California tightly restricted assault rifles, the suspect in that case legally bought one in Nevada, a state with comparatively lax gun laws. That phenomenon has been repeated in other shootings and states, as this analysis from FiveThirtyEight shows.

Some states that have imposed their own assault weapon bans have also experienced widespread noncompliance. Only approximately 45,000 of an estimated 1 million weapons were registered in New York during the first 18 months its law was in effect. In California, 18 months after legislators expanded its criteria for assault weapons registration in 2016, only about 160,000 of an estimated 1.5 million weapons in the state covered by the law were registered. The state bans have been further undermined by gunmakers who make weapons that comply with the letter of the law, but are functionally similar to the restricted weapons.

What factors would a new federal assault weapons ban have to contend with?

Even when the original was passed, its exemption for existing assault weapons amounted to a giant loophole. In the years since it lapsed, assault weapons have exploded in popularity.

Assault-style rifles have been available to consumers since the 1950s. But, comparatively speaking, the guns were not especially common before the federal ban lapsed. According to the National Shooting Sports Foundation, two thirds of the 8.5 million AR- or AK- style rifles that entered the civilian market between 1990 and 2012 became available after 2004. There are now approximately between 15 and 20 million assault weapons in Americans’ hands, according to The Trace’s analysis of existing estimates.

An updated assault weapons ban proposed by Representative Cicilline contains two provisions that seek to address some of these issues: While also including a grandfather clause for existing assault weapons, it would mandate background checks for selling or transferring them, even in transactions between private parties. It would also require that existing assault weapons be stored in a locked container or with a gun lock if an owner has reason to believe their weapon could be accessed by someone prohibited from possessing it — a nod to the fact that young mass shooters have frequently used guns taken from family members.

But Klarevas thinks those provisions don’t go far enough. To be effective, he argues, any new federal ban should not allow owners to sell or give away the assault weapons already in their possession. “You want to keep your assault rifle? Fine. But it’s yours only,” he said. “That way, you could eventually phase them out.”

What about the federal government buying back existing assault weapons?

According to The Trace’s 2020 campaign guide, three of the leading candidates for the Democratic presidential nomination are suggesting a voluntary version of the buybacks undertaken in Australia and, more recently, New Zealand. But those cases are difficult to compare to the American context: Australia’s buyback took in under a million firearms, a small fraction of the 15 to 20 million assault weapons Americans own. Politically, legally, and logistically, a buyback is a whole other can of worms. It is best parsed separately from a ban on production and sales.

OK, so are there other, more feasible alternatives to assault weapons bans that attempt to achieve similar goals?

Gun enthusiast and tech writer Jon Stokes has argued that the fundamental problem with assault weapons bans is that they don’t contend with the sheer prevalence of the weapons before they are enacted, or the ease with which small technical changes can evade the statutes. Instead, he has proposed a national licensing scheme for all semiautomatic guns.

A version of his idea already exists in 15 states, where residents are required to secure a gun ownership permit before purchasing firearms. Such systems have been associated with significant declines in gun deaths, both homicide and suicide. Stokes contends that his proposal would not be easily subverted by engineering tweaks, and would apply to the handguns commonly implicated in the majority of gun deaths, not just mass shootings.

Fordham University historian Saul Cornell has an alternative suggestion. Writing in The Atlantic, he advocated for heavily taxing assault weapons and ammunition. Cornell’s proposal included tax breaks for gun owners who also own safes. He argued that a tax would reduce the number of such guns in circulation by increasing the cost of ownership.

Cornell writes, “Taxation offers a more flexible set of tools to achieve a goal all Americans seek: lowering the costs of gun violence to Americans.”