IT'S designed as a military killing machine and instead it's doing its job slaughtering the innocent in shopping malls, schools and cinemas across America.

The gun that's killing America is a lightweight, easy to handle assault rifle that has become a lethal weapon in the hands of losers, misfits and people with a grudge.

Gun enthusiasts call it the "iPhone" of gun shops.

From the Sandy Hook school massacre, to the Dark Knight Cinema slayings and now the Los Angeles Airport shootings last weekend, the AR-15 is the firearm of choice for people who want to wreak maximum havoc.

A civilian version of the legendary M-16 used by US troops in battle since the Vietnam War and widely adopted by armies around the world, the AR-15 is owned by more than three million Americans.

They are popular in American culture, the rifle in the hands of heroes and villains in films and video games.

Sold over the counter in discount chain stores like Walmart across the US, they are increasingly the firearm chosen by ordinary citizens to defend themselves.

They can be accessorised with extra scopes and larger clips to suit individual use.

"You see it and you want it," one customer told CBS News. "It's the same reason someone waits 24 hours in line to buy an Apple iPhone."

Republican Congresswoman Michelle Bachman, a keen hunter and hard right Christian, alarmed anti-gun lobbyists when she announced she "loved" her AR-15.

"I love it," Bachmann declared. "It's a great gun."

Built by the Armalite company as a small arms weapon for the United States armed forces, and sold to Colt, the AR-15 model weapon has proliferated in different versions and models produced by almost every arms company on the market.

"Designed specifically for lightweight mobility, speed of target acquisition, and potent firepower capability - the Colt Law Enforcement Carbine delivers," Colt advertises on its website. "This specially designed law enforcement weapon system features many of the combat proven advantages of the military Colt M4."

Its popularity has spawned dozens of YouTube videos on how to choose your AR-15, how to fire it, and with demonstrations of young women in short shorts giggling and loosing off round after round in target practice.

The fully automatic version can fire up to 800 rounds per minute, and instructive videos explain how to "bump fire", or repeat fire, a semiautomatic version to give the spray effect.

Following the Sandy Hook school massacre last December, US president Barack Obama preached to the American people about controlling the availability of assault-style weapons, particularly to the mentally ill.

America's powerful gun lobby, headed by the National Rifle Association continues to resist anti-gun efforts and the Republican-dominated Congress earlier this year rejected a key gun control measure that would have expanded background checks.

Far from discouraging Americans, the Sandy Hook massacre saw in its wake an escalation of sales with AR-15s "flying off the shelves" of gun shops.

Around 47 per cent of Americans own firearms, and they bought weapons in record numbers in 2012.

Here's how the AR-15 is killing Americans:

November 1, 2013, Los Angeles Airport

Last Friday Paul Ciancia, a 23-year-old who had been experiencing an urge to shoot and self- harm, walked into LAX's Terminal 3 at 9.20am local time and opened fire with an AR-15 assault rifle.

At point blank range, he shot dead a Transportation Security Administration (TSA) agent, Gerardo I. Hernandez, 39.

Evidence from Ciancia's relatives and police later revealed the young man had planned to murder "multiple" agents.

Describing how the shooting started, a prosecutor said Ciancia, dressed in black and wearing a bulletproof vest, had pulled out a ". 223-calibre M and P-15 assault rifle" out of his bag and fired multiple rounds.

Ciancia then proceeded further into the terminal, shooting four more people, including two other TSA officers, before being detained following a gunfight with airport police.

A note in Ciancia's bag allegedly said he had made a conscious decision to kill multiple TSA workers, for whom he felt "anger and malice".

A former classmate of Ciancia's said he was a loner and had been bullied at their private high school.

Sandy Hook, December 14, 2012

On December 14, 2012, 20-year-old Adam Lanza took his mother, Nancy's Bushmaster XM15-E2S rifle, a version of the AR-15, and killed her at their home.

Then he drove to Sandy Hook Elementary School and embarked on mass murder.

Stalking classrooms with the weapon and aiming at cowering pupils and teachers, he fatally shot twenty children and six adult staff members.

The killing spree only ended when police arrived and he committed suicide by shooting himself in the head.

It was the second deadliest mass shooting by a single person in American history, after the 2007 Virginia Tech massacre.

The shootings prompted renewed debate about gun control in the United States, and a proposal for new legislation banning the sale and manufacture of certain types of semiautomatic firearms and magazines with more than ten rounds of ammunition.

Laws remain the same.

December 11, 2012, Clackamas Shopping Mall, Portland, Oregon.

Wearing a white hockey mask and a load-bearing vest, and armed with the AR-15 he had stolen, Jacob Tyler Roberts stormed into Clackamas Town shopping centre, where around 10,000 people were clustered in the food court at around 3.25pm.

People who first saw him run into the mall thought he was dressed for paintball, and the rifle was a toy.

Roberts opened fire on the food court, first shooting a 54-year-old hospice nurse and mother of two, Cindy Ann Yuille, in the back.

Next was 15-year-old Kristina Shevchenko, who he shot through the chest, but who survived her wound.

Finally, Steven Forsyth, 46, a youth sports coach and father of two Roberts shot in the head and killed, after which Roberts committed suicide.

Roberts reportedly grew up not knowing his own father and his biological mother died from cancer when he was two years old.

He had plans of joining the United States Marine Corps, but had to abandon his dream after a bicycle accident.

Prior to the shooting, he had broken up with his girlfriend, sold his belongings, resigned from his job, and was allegedly moving to Hawaii.

July 20, 2012, Century Movie Theatre, Colorado

A midnight screening of the film The Dark Knight Rises was in progress at the Century cinema theatre in Aurora, Colorado, when through the door came a man dressed in black, wearing a gas mask, a load-bearing vest packed with ammunition, a ballistic helmet, bullet-resistant leggings, a bullet-resistant throat protector, a groin protector and tactical gloves.

Members of the audience appeared unalarmed initially; some were in costume for the screening.

Then the masked gunman threw two tear gas canisters, the smoke obscuring the audience members' vision, making their throats and skin itch and their eyes stream.

Then he began firing.

At first, he let off rounds from a 12-gauge Remington 870 Express Tactical shotgun, which boomed into the ceiling and then into the audience.

He next fired a Smith & Wesson version of the AR-15 - an M & P15 semiautomatic rifle with a 100-round drum magazine.

Finally, he fired a Glock 22 40-caliber handgun.

Twelve people died and 58 were wounded.

On September 16, 2013, Aaron Alexis, a lone gunman fatally shot twelve people and injured three others in a mass shooting at the headquarters of the Naval Sea Systems Command (NAVSEA) inside the Washington Navy Yard in Southeast Washington.

Although Alexis is now believed to have used a shotgun as his killing weapon, it was revealed he had rented an AR-15 semiautomatic rifle and used it for target practice at a Northern Virginia gun range and store less than two days before his shooting spree

Following the shooting, President Obama held a press conference.

After Sandy Hook, the president had said: "Our tears are not enough ... our words and our prayers are not enough. ... We are going to have to change.

"We can't tolerate this anymore. These tragedies must end. And to end them, we must change."

After Washington, he called for a "transformation" of the nation's gun laws, saying the deadly shooting at the Navy Yard echoed "too many other killings" across the United States.

Six weeks later, Paul Ciancia opened up at LAX.

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