One industry group estimates that U.S. citizens now own more than 8 million AR-15 rifles.
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Omar Mateen, the gunman who killed 49 people and wounded more than 50 others in the Orlando massacre, used a model of semi-automatic rifle similar to the AR-15 called a Sig Sauer MCX. He had bought it legally just days before.
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AR-15: $1,000 per gun
According to a 2010 National Shooting Sports Foundation study, the rifle costs, on average, $1,000. A CBS News producer was able to purchase one from a Virginia gun store along with 100 rounds of ammunition and a 30-round magazine for $1,030.
The entire transaction took 38 minutes.
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AR-15: Banning attempts
The weapon has become both a symbol of rampant violence and one of America's favorite guns.
In June 2016, the U.S. Supreme Court rejected a challenge to state laws banning assault weapons and large-capacity magazines in New York and Connecticut. Those laws were enacted in response to the December 2012 massacre of 20 children and six educators at Sandy Hook Elementary School in Newtown, Connecticut.
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AR-15: State bans
California, Hawaii, Maryland, Massachusetts, New Jersey and Washington, D.C., also have laws substantially restricting or banning such weapons, according to the Law Center to Prevent Gun Violence. In addition, Minnesota and Virginia have strong regulations for assault weapons, the center said.
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AR-15: Military style
The AR-15 is modeled after a rifle originally designed for military use. It was tweaked for the civilian market. "AR" stands for Armalite Rifle, the company that first developed it.
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AR-15: Hundreds of rounds in minutes
The rifle fires one round (or one single bullet) per pull of the trigger, but is capable of firing hundreds of rounds within minutes.
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AR-15: Make it your own
One reason for its popularity is its many options for customization. There are Pinterest and Instagram pages dedicated to how versatile the AR-15 is. In fact, it is sometimes called the Lego set of the arms world.
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Massacre guns bought legally
Every gun used in the shootings at Orlando, San Bernardino, Newtown and Aurora was bought legally.
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AR-15: Purchases surge
In fact, directly following a number of highly publicized shootings, people have rushed to stores to buy these rifles (and firearms in general), afraid of more attacks or that lawmakers will curtail their sale. In January 2013, the month after the attack on Sandy Hook Elementary School and President Obama's call for greater restrictions on the sale of assault weapons, a record-breaking 2 million guns were sold in America, according to a New York Times analysis of federal data.
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AR-15: Federal ban
In 1994, Congress passed and President Bill Clinton signed a federal assault weapons ban that included some models of the AR-15. The ban expired in 2004.
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AR-15: Walmart discontinues
Walmart, the nation's leading gun dealer, stopped selling the AR-15 and a range of other military-style weapons in 2015. The company reportedly said it was due to declining consumer demand and was not a political statement.
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AR-15: Newtown lawsuit
In April 2016, a Connecticut Superior Court judge ruled that a lawsuit can go forward against the maker of the rifle used in the 2012 Sandy Hook Elementary School shooting. Families of the Newtown victims who filed the lawsuit argue the AR-15 is a military weapon and should not have been sold to civilians.
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AR-15: More powerful weapons
"Gun companies heavily market increasingly more powerful weapons," said Daniel Webster, professor and director of the Johns Hopkins Center for Gun Policy and Research. "There is constantly something new and more powerful on the market to entice buyers."
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AR-15: Handoff between friends
In the summer of 2016, the Oregon State Police said it would investigate the case of Jeremy Lucas, a Lake Oswego pastor who had entered a raffle to win an AR-15 in order to destroy it. Lucas apparently transferred the prize gun to a friend for safekeeping without performing a background check-a requirement of Oregon state law.
Lucas has reportedly said he's not worried about the investigation and would cooperate if asked.
AR-15: Buy it on Facebook?
In August 2016, BuzzFeed reporter Alex Kantrowitz published an essay claiming he'd "found, negotiated for, and agreed to purchase" a variation of the AR-15 entirely through Facebook.
Kantrowitz said he'd joined a gun-themed group on the site; found a post about the gun; and signed the paperwork to purchase it.