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

Five of the six deadliest mass shootings of the past six years in the United States. In each of them, the gunman had an AR-15-style semi-automatic 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 M16 and the shorter M4 carbine.

It was easily adapted for civilian use, with one major difference: Military versions can fire fully automatically - or in bursts of several shots - by depressing the trigger once. The civilian semi-automatic 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 man accused of the killings in Parkland 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 very easy to buy - and for a 19-year-old like Nikolas Cruz, the shooting suspect, far easier to obtain than a handgun.

While 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 semi-automatic 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 J. Forcelli, special agent in charge for the Bureau of Alcohol, Tobacco, Firearms and Explosives in Miami.

Congress explicitly banned AR-15s and other semi-automatic 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 - an exemption known as the “gun show loophole.”)

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.

“These guns are much more deadly than traditional bolt-action rifles or traditional shotguns, but they are regulated the same way” in most of the country, said Adam Skaggs, chief counsel of the Giffords center.

Among the six deadliest mass shootings in the last six years, the only one not carried out with an AR-15-style rifle was the Orlando, Florida, nightclub attack two years ago that left 49 dead.

The gunman, Omar Mateen, had a Sig Sauer MCX semi-automatic rifle, which shares features with the AR-15, though it functions via a different semi-automatic design - and is just as deadly.

All of these military-style semi-automatic weapons have something else in common. They have been heavily marketed as home-defence and marksmanship weapons, and their sales have been a major driver of profits for gun manufacturers over the past two decades.

“Americans own about 5 million AR-15s and it should go without saying that virtually all AR-15s are never misused,” a post on an NRA website said a few years ago, describing home protection and shooting competitions as among the ways the guns “are used by good people for perfectly good reasons.”

The industry also now calls these weapons “modern sporting rifles.” Gun organisations detest the label “assault rifle,” even though the federal legislation that enacted the decadelong ban specifically described the AR-15 and similar models as assault weapons. (The “AR” in the name, however, does not stand for assault rifle - it actually stands for ArmaLite rifle, reflecting the name of its original manufacturer.)

Paradoxically, the industry has boomed when under threat of new regulations, as buyers made new purchases because they feared additional ownership limits were about to be imposed.

For similar reasons, the election of Donald Trump helped clobber the share prices of some gun makers, as fears of new restrictions eased.

Florida shooting – in pictures Show all 10 1 /10 Florida shooting – in pictures Florida shooting – in pictures Police arrest a suspect in connection with the shooting at the Marjory Stoneman Douglas High School in Parkland, Florida Reuters Florida shooting – in pictures Parents wait for news after reports of a shooting at Marjory Stoneman Douglas High School in Parkland, Florida AP Florida shooting – in pictures Anxious family members wait for news of students AP Photo/Wilfredo Lee Florida shooting – in pictures Students being evacuated from the school AP Florida shooting – in pictures Students being evacuated from the school Getty Florida shooting – in pictures People gather waiting for word from students AP Florida shooting – in pictures Parents waiting for news on their children AP Florida shooting – in pictures People gather at a hotel where students were taken after the shooting Getty Florida shooting – in pictures Florida Governor Rick Scott speaks to the media as he visits Marjory Stoneman Douglas High School following the shooting AFP/Getty Florida shooting – in pictures Dr. Igor Nichiporenko, Medical Director Trauma, left, and Dr. Evan Boyer, Medical Director, Emergency Services, speak about treating victims and the suspect at a press conference outside Broward Health North hospital AP

Publicly-traded gun makers Sturm, Ruger & Company and American Outdoor Brands both lost about a quarter of their market value in the days after Trump’s victory.

Earlier this week the Remington Outdoor Company, one of the country’s biggest and oldest gun manufacturers, said it would seek Chapter 11 bankruptcy protection amid plunging sales and mounting losses.