When an 18-year-old shooter arrived last week at Robb Elementary School, in Uvalde, Texas — where he ultimately killed 21 people, including 19 children and two teachers — he carried 1,657 rounds of ammunition, authorities said.
The large number of rounds should not come as a surprise, experts told ABC News. The tragedy drew renewed scrutiny to a collection of state and national laws that regulate ammunition less tightly than firearms, despite the vital role played by ammunition in mass shootings, experts said.
A shooter at a Las Vegas music festival, in 2017, who killed 59, had at least 1,600 rounds. A shooter at an elementary school in Sandy Hook, Connecticut, in 2012, who killed 27, had more than 1,700 rounds of ammunition at his home. And a shooter at a movie theater in Aurora, Colorado, also in 2012, who killed 12, had bought more than 6,000 rounds, officials said.
Current regulations often allow for the purchase of massive amounts of ammunition and high-capacity magazines without a background check or even a face-to-face interaction, experts added.
While gun control proponents say ammunition deserves stronger restrictions that would limit the frequency and severity of mass shootings, gun rights advocates argue that ammunition restrictions violate Second Amendment protections and unnecessarily duplicate the regulations in place for guns, experts said.
"There are fewer restrictions on ammunition sales than there are on firearm sales both at the federal level and in the vast majority of states," Jacob Charles, executive director of the Center for Firearms Law at Duke University School of Law, told ABC News.
"Someone intent on a mass casualty event is going to have enough ammunition to be able to keep shooting until they're stopped," he said.
Federal law prohibits the sale of ammunition in a narrow set of circumstances, experts said.
People cannot purchase or possess ammunition if they've been convicted of a felony or misdemeanor domestic violence, committed to a mental institution, or if they belong to a handful of other categories deemed at-risk, experts said.
Also, under federal law individuals must be at least 18 to buy rifle or shotgun ammunition, and at least 21 to buy ammunition for any other guns. On top of that, in 1986, the U.S. enacted a law that bans armor-piercing bullets, which became notorious for the threat they posed to police officers.
Absent from national regulations are measures that require background checks for the buyers of ammunition or licenses for the sellers, which undermines enforcement of the few federal laws that are on the books, Tom Donohue, a law professor at Stanford University who specializes in gun legislation, told ABC News.
"Any restrictions on ammunition at the federal level are virtually feckless because you don't have to go through a background check to purchase ammunition," he said.
Federal law also lacks a measure that addresses high-capacity magazines, which enable shooters to fire a large number of bullets without stopping to reload. Such a law did exist once at the national level: The assault rifle ban enacted by Congress in 1994, which lapsed 10 years later, included a ban on high-capacity magazines.
A study published in 2019 by three researchers, including David Hemenway, director of the Harvard Injury Control Research Center, examined mass shootings over a nearly three-decade period and found that attacks involving high-capacity magazines had a 62% higher average death rate than those without them. The study also showed that high-fatality mass shootings occurred more than twice as often in states without bans on high-capacity magazines than in states with them.
In all, nine states have enacted bans on high-capacity magazines, including predominantly Democrat-controlled states like New York and Connecticut, according to the Giffords Law Center to Prevent Gun Violence.
The landscape of high-capacity magazine bans reflects a general trend of state-level measures that strengthen ammunition laws in a small group of mostly blue states, while the remainder of states go no further than federal law, Charles said.
For instance, New York and California have instituted mandatory background checks for ammunition purchases at the time a sale takes place, Charles added.
Gun rights advocates staunchly oppose ammunition regulation, experts said. Gun proponents argue that there's no need for additional regulation of ammunition once an individual has been deemed fit to own a gun.
"In theory, if I have given you a license and found you to be a law-abiding citizen, there's no reason for me to care what kind of gun you buy or how much ammunition you buy or what else you do, as long as it's legal," Alexandra Filindra, a political science professor at the University of Illinois, Chicago who studies gun laws, told ABC News.
Similarly, gun rights advocates have argued that regulation of ammunition infringes on their Second Amendment protections in the same manner that gun regulations do, since ammunition is a necessary part for operating a gun, Filindra said.
"The theory goes that essentially you are implicitly regulating gun ownership by taking away people's ammo," she said.
A network of gun rights groups, most notably the National Rifle Association, has fought ammunition regulation by framing it as an attack on gun ownership, the experts said.
"The same group standing in the way of gun safety reform is standing in the way of ammunition reform," said Ari Freilich, the state policy director at Giffords Law Center.
Some ammunition-related bills have been introduced in Congress. The Age 21 Act, put forward by Senator Dianne Feinstein, D-Calif., would raise the minimum age for buying assault rifles and high-capacity magazines from 18 to 21. Meanwhile, a bill in the House would require a license for all ammunition sellers and mandate that all ammunition sales take place in person.
Experts said that meaningful reform of ammunition laws is unlikely in the short term, but some said that incidents like the mass shooting in Uvalde make action more likely in the long term.
"A lot of people have woken up to how senselessly, dangerously reckless our lack of protections currently are," Freilich said.