Jonathan Scalise, owner of Ammunition Storage Components, holds up a 10 round, left, and a 30 round, right, magazine that his company manufacturers for the AR-15 rifle in New Britain, Conn. (Charles Krupa/AP)

With President Obama’s push for limits on high-capacity ammunition magazines stalled, some states are restricting their size in a shift that authorities say could dramatically reduce deaths in future mass shootings.

Congress is set to debate expanded background checks for gun purchasers in the coming days, but Obama’s proposals to ban some assault-style weapons and impose a 10-round limit on magazines are unlikely to reach a vote.

Governors and legislatures across the country, however, are seizing the magazine issue. Rhode Island on Tuesday became the latest state to propose restrictions on high-capacity magazines. Connecticut and Maryland have banned the sale of magazines that hold more than 10 rounds of ammunition. California prohibits the sale of magazines holding more than 10 rounds and has proposed a ban on their possession. New York and Colorado also have adopted new restrictions.

Firearms experts say restrictions on magazines — detachable ammunition storage and feeding devices — might not prevent a fatal shooting but could prevent it from turning into a massacre.

“They will prevent a killer from becoming a killing machine,” said David Chipman, a former Bureau of Alcohol, Tobacco, Firearms and Explosives official who is a senior policy adviser for Mayors Against Illegal Guns. As a special agent arresting criminals, Chipman said, he routinely carried a firearm with a magazine that was limited to 15 rounds.

How the NRA exerts influence over Congress

Gun rights advocates oppose the limits, saying Americans need the increased firepower to defend themselves against criminals.

Lawrence Keane, senior vice president and general counsel at the National Shooting Sports Foundation, which represents firearms manufacturers and retailers, said studies show that the now-expired federal ban on assault weapons and high-capacity magazines did not reduce crime. He also argues that it would take a shooter less than a second to change standard magazines. The idea that restrictions on large magazines would save lives in shootings such as the one in Newtown, Conn., are “speculative at best,” he said.

“What stopped Adam Lanza is that the police arrived and he killed himself,” Keane said, referring to the gunman who killed 20 first-graders and six educators at a Newtown school in December.

Advocates of gun rights have had their victories, too. Several states have passed laws expanding the right to carry weapons.

The large-capacity magazines, which greatly increase a gun’s firepower, were designed for the battlefield to help soldiers spray a huge number of bullets quickly without reloading or being skilled marksmen. Some magazines hold 30 or 60 rounds; others, called “drums,” contain at least 100.

Magazines holding more than 10 rounds were banned from 1994 to 2004, when military assault weapons also were banned. Once the ban expired, sales of the large magazines soared. A Washington Post investigation found that the rate at which police recovered guns in Virginia with high-capacity magazines began to drop around 1998, four years into the ban and hit a low of 9 percent the year the ban expired. The next year, the rate began to climb, and it reached 20 percent in 2010.

High-capacity magazines are unregulated, with neither background checks nor records required. Michael Bazinet, a spokesman for the National Shooting Sports Foundation, said that about 130 million detachable magazines have been sold in total in the United States; more than 30 million of them hold 30 or more rounds of ammunition.

Current and former law enforcement officials say that Bazinet’s figure is conservative and that no accurate number is available.

“We have no more idea than the man in the moon how many are out there,” said Jim Pasco, a former ATF official who is the executive director of the national Fraternal Order of Police. “They are no more regulated than a Q-tip, and they historically haven’t been, so how could we know how many there are?”

Sen. Richard Blumenthal (D-Conn.), who is sponsoring legislation to limit magazines to 10 rounds, said that what happened at Sandy Hook Elementary School in Newtown shows that a limit would make a difference.

“The killer at Newtown shot 154 bullets in less than five minutes because he had numerous magazines with 30 rounds each,” Blumenthal said. “He was able to kill more people more quickly. When he had to change magazines after the first 30 rounds, children were able to escape from the classroom. The more time the shooter has to change magazines, the more people can escape and the more opportunities there are to stop him.”

James Holmes was armed with an assault-style weapon equipped with a drum-type magazine that held 100 rounds when he killed 12 people inside an Aurora, Colo., movie theater in July. Maj. Nidal M. Hasan, who killed 13 at Fort Hood in 2009, was armed with magazines that had 20-round and 30-round capacity.

In Tucson in January 2011, Jared Loughner used a 9mm semiautomatic handgun and a large-capacity magazine to fire 33 bullets in 15 seconds, killing six people and wounding 13, including then-Rep. Gabrielle Giffords (D-Ariz). Christina Green, 9, was shot after the 12th bullet. Loughner’s massacre ended only when he stopped to reload.

“He fumbled and dropped his next, fully loaded magazine, creating a moment for a brave bystander to tackle him,” said Pia Carusone, executive director of Americans for Responsible Solutions, the group created by Giffords and her husband, Mark Kelly.

In the next few weeks, Kelly and Giffords plan to urge legislators in several other states to restrict the size of magazines.

Tom Diaz, a former senior policy analyst at the Violence Policy Center, said high-capacity magazines have nothing to do with hunting.

“To kill a deer, you are only interested in taking one or two shots,” said Diaz, author of a new book, “The Last Gun: How Changes in the Gun Industry are Killing Americans and What It Will Take to Stop It.” “A 100-pound drum is not for sporting purposes. These high-capacity magazines were designed to shoot people on the battlefield, where the more bullets you put out over close to medium range, the more damage you are likely to inflict. They call it the bullet hose.”

Veronique Pozner, the mother of 6-year-old Noah, the youngest victim in the Newtown massacre, said she does not want to sugarcoat what happened to her son, who was shot with a semiautomatic rifle and a high-capacity magazine.

“We all saw how beautiful he was,” Pozner told an online publication, describing Noah in his coffin. “He had thick, shiny hair, beautiful long eyelashes that rested on his cheeks. He looked like he was sleeping.”

“But the reality of it was under the cloth he had covering his mouth there was no mouth left,” she said. “His jaw was blown away. I just want people to know the ugliness of it so we don’t talk about it abstractly, like these little angels just went to heaven. No. They were butchered. They were brutalized. And that is what haunts me at night.”

Julie Tate and Ed O’Keefe contributed to this report.