The AR-15: What is it and why is it so popular?

(RNN) - A common thread through many of America's recent mass shootings is the gun used: an AR-15 rifle, or one of its many variants.

The gun is one of the country's most popular.

It has been targeted by gun control advocates and fiercely defended by Second Amendment supporters. Here's a quick history of the firearm and why it may be so popular.

Where did the AR-15 come from?

The AR-15 was developed by Eugene Stoner for ArmaLite, who conceived it for the American military, at the time looking for a Cold War competitor to the lightweight, powerful AK-47 of the Soviets and their communist proxies. When ArmaLite failed to sell it to the Army, the struggling company sold it instead to Colt in 1959.

After Colt’s efforts to tweak the product and market it more widely, the Air Force began adopting the rifle in 1960. Despite influential opposition from some military leaders in favor of the established M14, a production shortage for that weapon in 1963 opened the door for the AR-15.

Colt made minor modifications at the request of the military, and the AR-15, now cleared for mass adoption, was rebranded the M16 – a defining weapon in the Vietnam War and America’s standard-issue rifle for nearly 50 years.

The original AR-15 was then fashioned into a semi-automatic weapon for the public market.

Why do so many people use the AR-15 today?

Its wide availability and accessible price help fuel its popularity. At $739 Smith & Wesson’s standard M&P15 Sport model based on the AR-15 sells in the same range as an iPhone. In the secondary and off-brand markets, they can cost as little as $400 or $500.

Crucially, it's also easily modified. This, in particular, makes it a favorite for hobbyists and supports a vast market for accessories. Big manufacturers and boutique outfitters alike offer a wide array of custom parts, from magazines to stocks to barrels.

And as a gun, it is compact, reliable, accurate and powerful.

Why does it show up in so many mass shootings?

There’s no one reason for that. But a likely cause is simply its combination of effectiveness and availability.

As it is considered the standard sport rifle on the market, it can be obtained through just about any gun-buying avenue, and the same traits that compelled the U.S. military to adopt it 50 years ago - portability, lethality - remain consistent today.

The weapon’s ability to be customized was exploited by Las Vegas shooter Stephen Paddock. He fitted legal AR-15s with legal bump stocks, an attachment that replaces a standard stock and effects repetition through a gun’s own recoil, generating significantly more rapid shooting than a finger manually pulling a trigger can.

The Bureau of Alcohol, Tobacco and Fire does not track arms sales in the U.S., but it does produce an annual report on manufacturing. The latest data showed there were more than four million rifles produced in America in 2016.

Part of it may ultimately come down to branding.

"It's really just a perception thing. There are rifles that are more powerful and more dangerous than that, but they're not being used," Dean Hazen, a gun shop owner in Illinois, told USA Today.

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