When Omar Mateen entered an Orlando nightclub to carry out the deadliest mass shooting in US history, he wielded a weapon that has been used in massacres from California to Connecticut: a military-inspired semi-automatic rifle.

Key points: Omar Mateen carried AR-15 and handgun, law enforcement officials say

Omar Mateen carried AR-15 and handgun, law enforcement officials say Opponents say civilians should not own "weapons of war"

Opponents say civilians should not own "weapons of war" There are 30,000 annual gun deaths in the US

Though so-called assault rifles account for a small fraction of the US's 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 US 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.

Law enforcement officials say Mateen carried an AR-15 style assault rifle and a handgun when he killed 49 people and wounded 53. He also had an unidentified device, Orlando Police Chief John Mina said.

The AR-15 was developed from the US military's M-16 rifle, used in the Vietnam War in the 1960s.

Sorry, this video has expired What happened in America's worst mass shooting

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 school shooting 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," Josh Koskoff, a lawyer involved in the case, said.

Gun used for target shooting, home defence

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

The NSSF estimates there are roughly 5 million to 10 million AR-15 rifles owned in the US — a fraction of the 300 million firearms owned by Americans.

Most owners say they use the rifle for target shooting and home defence, 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 per cent.

Roughly four times as many people were killed by knives in that period.

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 last year.

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

Reuters