What Are Assault Rifles?

A primer on America’s deadly obsession.

TRIGGER WARNING: This post contains detailed information on deadly weapons

Last Sunday’s horrible shooting at LGBTQ nightclub Pulse in Orlando, Florida, which left at least 49 dead and 53 wounded, has re-ignited a long-running debate over whether assault weapons should be outlawed.

“We have to make it harder for people who want to kill Americans to get their hands on weapons of war that let them kill dozens of innocents,” President Barack Obama told reporters after meeting with the National Security Council on Tuesday.

The National Rifle Association, the country’s largest and most powerful gun rights group, responded instantly.

"What’s obvious is that many who want to destroy our firearm freedoms are using the terrorist attack in Orlando to push their anti-gun agenda,” the group said in a statement. “They’d rather blame you as a law-abiding gun owner than make Obama answer for his failures in the global war on terror.”

Initially, authorities said the killer, 29-year-old Omar Mateen, was carrying an AR-15-style rifle and a handgun. We now know that Omar actually used a Sig Sauer MCX. The guns look similar but have different firing mechanisms.

Both are often classified as assault weapons and now Congress is considering a new ban on these kinds of firearms. But what is an assault, or semi-automatic, weapon (the two terms are often used interchangeably), and why are they so controversial? Let’s take a look.

How Guns Are Classified

Most handguns and rifles fit into one of three broad categories: non-automatic, semi-automatic, and fully automatic.

With non-automatic weapons, the user must manually cycle the gun to the next round (called "cocking" the gun), and then pull the trigger. For semi-automatic weapons, they cycle automatically to the next round, but the user must still pull the trigger each time. Fully automatic weapons (often just called “automatic,” but sometimes called “machine guns”), on the other hand, cycle from round to round automatically, and the user does not have to pull the trigger for each shot fired. This is why they have the potential to cause so much more harm.

Both non-automatic and semi-automatic guns are legal in the United States. Fully automatic weapons are illegal for civilian use in the U.S. under the Firearm Owners' Protection Act, signed into law by President Ronald Reagan in 1986.

What Is an Assault Weapon?

Large, military-style rifles are often called “assault weapons,” though the gun industry calls them “modern sporting rifles.” These weapons were originally designed for military use, and they’re built for combat. Many were illegal in the U.S. from 1994 to 2004 under the Federal Assault Weapons Ban. Since the ban expired in 2004, they’ve become both popular and controversial.

Here’s the Justice Department’s definition of an assault weapon, according to CNN: “In general, assault weapons are semiautomatic firearms with a large magazine of ammunition that were designed and configured for rapid fire and combat use.”

Assault rifles, like the popular AR-15, usually fire smaller bullets than handguns. But they’re capable of firing many more bullets without reloading, which makes them potentially more lethal.

The AR-15 is the most popular assault rifle in the U.S., and was originally designed to give U.S. soldiers an advantage over Russian-made AK-47 rifles during the Vietnam War. The M-16, a military adaptation of the gun, was the standard-issue U.S. military infantry weapon from 1969 to around 2011, when it was replaced.

California, Connecticut, Hawaii, Maryland, Massachusetts, New York, and New Jersey have their own state-level assault weapons bans. As The San Diego Union Tribune pointed out in 2013, there’s no one definition of an assault rifle. Relatively minor changes can make the same weapon fit into the category or not. And while some of those changes could make the weapon less deadly, not all do.

Why Ban Assault Rifles

Since the assault weapons ban expired in 2004, there have been 47 mass shootings in the U.S. that left 411 people dead, according to a Boston Globe editorial calling for a return of the assault weapons ban.

The system of background checks meant to keep guns out of the hands of people who might be dangerous is also notoriously flawed. And private sellers, like many of those at gun shows, don't have to do background checks at all.

Assault weapons have been used in high-profile mass shootings like Sunday’s in Orlando, along with one at a holiday party in San Bernardino, California, last December that left 14 dead; one at Sandy Hook Elementary School in Newtown, Connecticut, in 2012 that killed 20 6- to 7-year-old children and six adults; and one at a movie theater in Aurora, Colorado, that left 12 dead.

On Thursday, the family of Eugene Stoner, who designed the AR-15 and M-16 rifles, told NBC News that their father never owned either rifle and never intended them for widespread civilian use.

“He died long before any mass shootings occurred,” the family said. “But, we do think he would have been horrified and sickened as anyone, if not more by these events.”

Related: Why Suspected Terrorists Are Legally Allowed to Buy Guns

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