Owned by 5 million Americans, AR-15 under renewed fire after Orlando massacre

It's a popular rifle among U.S. gun owners. And it's become especially popular as the weapon of choice in American mass shootings.

The massacre in an Orlando, Florida, nightclub that left 50 dead and dozens more wounded has once again prompted calls for a ban on the AR-15, the weapon used in the worst mass shooting in modern U.S. history.

The Orlando massacre "is more horrific evidence of the unique lethality of the AR-15," Joshua Koskoff, a lawyer representing the families of those killed and injured in Sandy Hook, Connecticut, said in a statement. "It is no wonder that this weapon was chosen by today's shooter, as it has been by so many before him, and as it undoubtedly will be again."

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

The latest massacre also elevated a long-running political debate about banning so-called assault-style weapons for civilian use.

The Orlando massacre "reminds us once more that weapons of war have no place on our streets," Democratic presidential candidate Hillary Clinton posted on Facebook on Sunday.

The National Shooting Sports Foundation, which represents gun manufacturers, declined a request for an interview "until more information is known."

The AR-15, developed from the U.S. military's M-16 rifle, is manufactured by dozens of U.S. companies, including major gun makers like Smith & Wesson, Sturm Ruger and Remington Arms. Unlike the military version, the AR-15 is not fully automatic, meaning the shooter must pull the trigger each time to fire a shot. But like the military version, the guns are capable of firing many rounds of ammunition quickly, and can be reloaded rapidly.

Sales have been brisk for U.S. gun makers in the last decade. The number of total firearms made topped 10 million in 2013, the latest data available from the Bureau of Alcohol Tobacco and Firearms. That's more than double the industry's output in 2008.

In the past 10 years, assault-style rifles have been used in 14 public mass shootings, Reuters reported, Half of those shootings have occurred since last June, according to a data base maintained by Mother Jones magazine.

Assault-style weapons were banned in 1994, but that prohibition expired in 2004 and Congress opted to not renew it. Gun-rights advocates note that rifles of any type are rarely used in U.S. homicides. FBI data show that in the five years ending in 2014 handguns accounted for at least 48 percent of all murders while rifles, including more traditional types of long guns, accounted for 2.4 percent.

"Americans own about five million AR-15s and it should go without saying that virtually all AR-15s are never misused," the NRA Institute for Legal Action says on its website. "Many are kept for home protection, particularly now that carbine versions are available in many configurations suited for defense in the close spaces of a home, often in low light conditions."

Exact numbers are hard to come by. The ATF doesn't break out AR-15 ownership in the registration data it makes public. The National Shooting Sports Foundation estimates there are roughly 5 million to 10 million AR-15 rifles owned in the United States, a small share of the roughly 300 million firearms owned by Americans.

The federal data do break out state-level gun registrations by category, including "machine guns," which are fully automatic weapons capable of continuous firing.

As of February 2015, Florida had the second-highlight number of registered machine guns, per capita, among the 50 states. Texas ranked first.

Reuters contributed to this report.