Florida school shooting: Why the AR-15 has become mass murderers', and America's, favourite weapon

The US has an enduring fascination with the AR-15. The interest isn't dulled when the gun is used in the US's mass shootings – every time there is such a killing, panic buyers grab the guns in bulk, and some reports even suggest that fans are motivated in part through the fascination with a weapon that could be used in such a horrific way.

The AR-15 was introduced by Colt in the early 1960s, explicitly aimed as a way of getting deadly assault rifles into the hands of civilians and law enforcement. Getting guns that were previously seen as military weapons into the hands of civilians was a key way of boosting sales – and it worked, with guns like the AR-15 and similar assault rifles becoming hugely popular.

The gun and its renown hasn't much changed since then, and has gone on to become one of the most popular weapons in the US and easy enough to buy that you could pick it up in Walmart. But it has gradually picked up a reputation for not only being America's favourite gun but also the favoured killing machine of its mass murderers.

Since then, it has gone to be a shockingly common feature of America's mass killings. Most recently, it was used by the shooter who carried out the deadliest high school shooting in modern US history at the Marjory Stoneman Douglas High School in Florida.

But it was far from the first time the weapon has been used in such a way, and it has a place in almost all of the US's most deadly and famous mass shootings. The Las Vegas gunman used one, modified so that it could shoot like a fully automatic gun; the Pulse nightclub shooter used a rifle in the same style to kill 49 people in Florida; it was used in the San Bernardino, Sandy Hook, and Colorado cinema shootings too.

The association with mass shootings might now be sticking, and encouraging murderers to use the gun precisely because it has been associated with previous killings.

Dean Hazen, a firearms instructor, told USA Today that he thought the gun was getting a "bad rap" and that it wasn't necessarily the most effective way of arbitrarily killing people as quickly as you can. "Thank God they don't know any better because if they did they would use much more effective weapons," he said.

The AR-15 was already incredibly popular and breaking through records before it was used in the shooting at Sandy Hook that made it infamous. But since then it has gone on to see huge sales, with rumours that it could be banned driving sales to unbelievable levels.

Now the gun – known as the "black rifle" to its fans – is in the hands of millions of people. They can be bought with relatively cursory background checks – available in Walmart until 2015, when nationwide revulsion at the gun led it to drop sales of the weapon, though it still sells accessories for the AR-15 alongside other assault rifles – despite their immense power.

Along with that power has come a whole industry of people devoted to admiring, buying and altering their guns. Companies sell a whole range of accessories and that modular approach is what makes it so spectacularly attractive both to mass murderers and gun fans who consider themselves safe.

One of the gun's most attractive features is that it can be easily modified, according to the NRA. That's also what makes it so much more attractive to mass murderers, too: it is easy to buy and then change into something even more powerful, without breaking laws that prohibit the manufacturing of fully automatic weapons for the public.

"Civilians can also modify and personalize their AR-15 from carbine-length, stocks, optics, barrels, etc. The AR-15s ability to be modified to your own personal taste is one of the things that makes it so unique."

It was one such modification, for instance, that allowed the Las Vegas shooter to kill so many people in such a small amount of time. By fixing an entirely legal "bump stock" to the gun, he was able to effectively turn it into a fully automatic weapon.

Modified or not, the gun looks exactly as you'd expect: if you were asked to draw an assault rifle, the AR-15 is the shape you'd probably see. That's partly because the gun itself is so prevalent, but also because it is now military issue.

The US armed forces use a fully automatic gun called the M16, which is actually just a redesigned version of the original AR-15. Though the two guns have different internal features, they are nearly identical to look at.

It's the use of similar weapons by the military that has led the AR-15 to become so beloved by organisations like the NRA and by ordinary gun fans. Specifically, it has been put down to specific special operations parts of the military – those groups that became famous after 9/11 and as a result of the games like Call of Duty that glorified their work.

At the same time, in that nervous era after 9/11, the Federal Assault Weapons Ban expired in 2004. That made it far easier both to sell and market the guns – removing restrictions that kept the AR-15 from being sold as a killing machine rather than a hunting weapon.

Despite calls each time there is a mass shooting, there is little suggestion that another piece of legislation of that kind could ever be passed. While many Democrats have supported it – including Barack Obama, when he was president, in the wake of the Sandy Hook shooting – it's not likely that it would be able to get through either the courts or the US Congress, as well as the assorted lobbyists encouraging them not to pass gun control.