Newtown. San Bernardino. Las Vegas. Sutherland Springs. And now, Parkland.

Five of the six deadliest mass shootings of the past six years in the U.S. In each of them, the gunman had an AR-15-style semiautomatic rifle.

The NRA calls the AR-15 the most popular rifle in America. The carnage in Florida on Wednesday that left at least 17 dead seemed to confirm that the rifle and its variants have also become the weapons of choice for mass killers. It is not hard to see why. Originally designed for troops to kill enemy fighters, the weapon became the military's M-16 and the shorter M4 carbine.

It was easily adapted for civilian use, with one big difference: Military versions can fire fully automatically — or in bursts of several shots — by depressing the trigger once. The civilian semiautomatic version requires a pull of the trigger for each shot.

But other features that make the AR-15 so deadly on the battlefield remain. It is light, easy to hold and easy to fire, with a limited recoil. Bullets fly out of the muzzle more than twice as fast as most handgun rounds.

Equally important for a gunman looking to do a lot of damage in a hurry: AR-15-style weapons are fed with box magazines that can be swapped out quickly. The standard magazine holds 30 rounds. Equipped in this way, a gunman can fire more than a hundred rounds in minutes.

The Parkland shooter had "countless magazines" for his AR-15, the local sheriff said. And there is still one more reason the weapons are so popular in states like Florida: They are easy to buy — and for Nikolas Cruz, 19, the shooting suspect, far easier to obtain than a handgun.

Florida has a three-day waiting period for handgun purchases. But anyone without a felony record, domestic abuse conviction or a handful of other exceptions — such as a commitment to a mental institution — can walk into a gun store, wait a few minutes to clear a background check, and walk out with an AR-15-style rifle, magazines and ammunition.

Under federal law, you also must be 21 to buy a handgun from a firearms dealer. But 18-year-olds can buy semiautomatic rifles.

The AR-15 rifle used in the attack was purchased legally at Sunrise Tactical Supply in Florida, according to a federal law enforcement official. "No laws were violated in the procurement of this weapon," said Peter Forcelli, special agent in charge for the Bureau of Alcohol, Tobacco, Firearms and Explosives in Miami.

Congress explicitly banned AR-15s and other semiautomatic rifles that fit its definition of assault weapons from 1994-2004.

Since that law expired 14 years ago, in most states it has been just as easy to buy an AR-15-style gun as in Florida. (Buyers can also purchase the weapon in person or online from private sellers who are not required to perform a background check — the "gun show loophole" exemption.)

Only New York, California, Washington, D.C., and five other states have their own assault-weapon bans, according to the Giffords Law Center to Prevent Gun Violence. Washington, D.C., and eight states also limit the large capacity magazines that have made many mass shootings so deadly. Even so, there have been efforts to circumvent the bans, legally and illegally, by making minor changes to the weapons.

Minnesota doesn't have a three-day waiting period for firearms. But for those who want to buy a handgun or certain rifles, including the AR-15, the state requires a permit to purchase, which includes a background check by local law enforcement.

Staff writer Mary Lynn Smith contributed to this report.