This is the powerful gun that has featured in almost every American mass shooting

Omar Mateen's murderous rampage in an Orlando LGBT nightclub shattered a record with an astounding 50 deaths by shooting, but something was also very familiar about the attack to Americans: the common assault-style rifle he used.

Such powerful weapons have become the gun of choice for American mass shooters. The rifles were used in 2015 in San Bernardino to kill 14, in 2012 in a Colorado movie theatre in a rampage that killed 12, and in a Newton, Connecticut, elementary school where 28 people, almost all of them young children, were murdered.

A man was recently caught driving to the Los Angeles Pride festival in a truck loaded with the rifles, said police.

The guns are produced for civilians and are based on military guns. Most are referred to as AR-15s, and can be semi-automatic or fully automatic, though only the semiautomatic version of the guns are legal in the US.

Mateen purchased his AR-15 legally, according to officials, even though he had been questioned twice by the FBI.

"He is not a prohibited person, so he can legally walk in to a gun dealership and acquire and purchase firearms," said an official at a Florida press conference. "He did so. And he did so within the last week or so."

Of eight high-profile mass shootings in the US in the last year, seven involved an assault-style rifle, according to Mother Jones Magazine — and their use is on the rise.

If America's guns law have an Achilles heel, this weapon would seem to be the most vulnerable to being rendered illegal. While the US Constitution references a right to bear arms, it does not specify that citizens need a gun that is capable of firing a large number of rounds in an extremely short period of time.

Assault-style weapons were outlawed in 1994 but when the ban expired in 2004, Congress opted to not renew it. At that point, gun supporters argued that any kind of rifle was rarely used to kill people and the ban didn't have an appreciable affect on the homicide rate, noted the Washington Post. While they continue to account for only a fraction of the annual American deaths by guns, they are increasingly responsible for mass shootings.

Al-Qaeda has viewed the legal existence of the weapons as an opportunity in the US. In 2011 a spokesman encouraged followers to buy the rifles in the US — and put them to use.

"America is absolutely awash with easily obtainable firearms," American-born al-Qaeda spokesman, Adam Gadahn, said in a video. "You can go down to a gun show at the local convention centre and come away with an ... assault rifle, without a background check, and most likely without having to show an identification card. So what are you waiting for?"

Of the 79 mass shootings since 1982 that Mother Jones was able to track, 63 were committed with guns that were purchased legally.

President Obama has called the shooting "a further reminder of how easy it is for someone to get their hands on a weapon that lets them shoot people in a school, or a house of worship, or a movie theater, or a nightclub."

He reiterated a need for stricter gun laws, saying, "To actively do nothing is a decision as well,"