WASHINGTON (Reuters) - When Omar Mateen entered an Orlando, Florida, nightclub on Sunday to carry out the deadliest mass shooting in U.S. history, he wielded a weapon that has been used in massacres from California to Connecticut: a military-inspired semi-automatic rifle.

AR-15 rifles line a shelf in the gun library at the U.S. Bureau of Alcohol, Tobacco and Firearms National Tracing Center in Martinsburg, West Virginia December 15, 2015. REUTERS/Jonathan Ernst

Though so-called assault rifles account for a small fraction of the United States’ 30,000 annual gun deaths, they have been used in at least 10 mass shootings since 2011, according to a database compiled by Mother Jones magazine.

The prevalence of these firearms has made them a focal point in the debate over U.S. gun laws as opponents say civilians should not own what they describe as “weapons of war.” Backers say they are simply modern rifles enjoyed by millions of law-abiding Americans.

In December 2012, Adam Lanza used a Bushmaster XM15 to kill 28 children and adults at the Sandy Hook elementary school in Connecticut before taking his own life with a Glock pistol.

Tashfeen Malik and Syed Farook used two assault rifles and two pistols to kill 14 people in San Bernardino, California, in December 2015.

James Holmes carried an assault rifle, a shotgun and two pistols when he killed 12 people in a Colorado movie theater in 2012.

Law enforcement officials say Mateen, a 29-year-old U.S. citizen who was the son of Afghan immigrants, carried an AR-15 style assault rifle and a handgun when he killed 50 people and wounded 53 at a gay nightclub in Orlando. He also had an unidentified device, said Orlando Police Chief John Mina.

The AR-15 was developed from the U.S. military’s M-16 rifle, used in the Vietnam War in the 1960s. Unlike the military version, the AR-15 is not fully automatic, meaning users must pull the trigger each time they want to fire a shot. Like the military version, many AR-15s combine light weight with a relatively modest recoil.

Prominent manufacturers include Smith & Wesson, Sturm Ruger and Remington Arms Co, which faces a lawsuit from some families of Sandy Hook victims who say the rifle should not be sold to civilians.

“It is the gold standard for killing the enemy in battle, just as it has become the gold standard for mass murder of innocent civilians,” said Josh Koskoff, a lawyer involved in the case.

The National Shooting Sports Foundation, which represents gun manufacturers, said it would not comment on the Orlando shooting until more facts are known.

TARGET SHOOTING AND HOME DEFENSE

The NSSF estimates there are roughly 5 million to 10 million AR-15 rifles owned in the United States, a fraction of the 300 million firearms owned by Americans. Most owners say they use the rifle for target shooting and home defense, although they can be used for hunting as well.

Despite their controversial reputation, assault rifles do not often turn up at murder scenes. Handguns accounted for at least 48 percent of all murders between 2010 and 2014, according to FBI data, while rifles - a category that includes more traditional types of long guns - accounted for 2.4 percent. Roughly four times as many people were killed by knives in that period.

Still, in the wake of the Orlando shooting several Democratic politicians said Congress should renew a ban on assault-style weapons and large-capacity ammunition magazines that was in place between 1994 and 2004.

“It reminds us once more that weapons of war have no place on our streets,” said presumptive Democratic presidential nominee Hillary Clinton.

Research is mixed on whether the assault-rifle ban had a significant impact. Gun manufacturers were able to tweak their designs to get around the ban and crime rates have fallen steadily since the early 1990s, whether or not the ban was in place. The number of mass shootings has increased since it expired.

Any effort to renew the ban would face fierce opposition in the U.S. Congress, where Republicans who control both chambers of Congress have staunchly opposed tighter gun laws and groups like the National Rifle Association have been able to mobilize millions of gun owners against other restrictions. Despite strong support from President Barack Obama, the last attempt at new gun legislation failed in the U.S. Senate in 2013.

Opinion polls find that Americans are less enthusiastic about banning assault weapons than other types of gun restrictions, such as expanded background checks. Several polls since 2013 have found between 44 percent and 57 percent of Americans back the idea.

Six states and the District of Columbia currently ban assault weapons and two more, Minnesota and Virginia, ban possession by people under 18 years old. Experts say it is difficult to tell whether those laws have had an impact since gun buyers can easily find those weapons in other states.