The AR-15 semi-automatic rifle used in Sunday's massacre has been used in several mass shootings since 2012.
Sandy Hook, San Bernadino and Aurora, Colorado, are just some of the mass shootings where the gunman was armed with an AR-15.
Five to 10 million Americans own this type of rifle, according to the National Shooting Sports Foundation.
The AR-15 was developed from the military's M-16 rifle. The difference between the two is that the AR-15 is not fully automatic.
Jason Wagner, chief operating officer at Frontier Justice in Lee's Summit, said there seems to be some confusion about the AR-15.
"It's portray that 'AR' stands for automatic rifle, when in reality it stands for Armalite Rifle," Wagner said.
Because the AR-15 is a semi-automatic rifle, the shooter must pull the trigger with every shot. The military rifle, the M-16, fires continuously - something everyday citizens don't have easy access to.
"You cannot legally own one of those," Wagner said.
Wagner could not speculate as to why this type of rifle is linked to so many mass shootings. He does say, however, that everyday people choose to buy this rifle for various reasons.
"You'll see an AR-15, not only used in home defense, but the recreation world," he said.
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