Used Military rifles

Here's what you need to know about the guns some are calling "the gold standard for mass murder, " after Orlando shooter Omar Mateen used the assault-style rifle Sig Sauer MCX to kill at least 49 people. (Editor's note: This video has been updated with more specific information.) (Gillian Brockell/The Washington Post)

Editor’s note: An earlier version of this story’s headline incorrectly identified the weapon used in Sunday’s shooting in Orlando as an AR-15. Orlando Police officials first classified the weapon used in the rampage as an “AR-15-type assault rifle.” On Monday, officials said the weapon used was a Sig Sauer MCX. While in many ways similar to the AR-15 family of rifles, the MCX relies on a different gas system to operate and cannot be fairly classified as an AR-15. Both weapons can fire the same type of ammunition at roughly the same speeds and share a similar history.

The type of rifle used to kill 49 people and injure 53 at an Orlando nightclub Sunday — in the worst mass shooting in U.S. history — is as American as baseball cards and apple pie.

Manufactured by dozens of companies nationwide, the ubiquitous synthetic and aluminum rifle, known as the AR-15, is a civilian variant of the military’s M-16 series of rifles and carbines. It’s favored because of its relative light weight and anchor points for numerous modifications, and serves as a type of lethal erector set for millions of Americans.

The AR-15 is synonymous with the shootings in San Bernardino, Calif.; Aurora, Colo.; and Newtown, Conn. Its design and likeness appear in video games, movies, television shows and toy stores. Military variants of the weapon have been shipped overseas en masse, arming U.S. allies, and sometimes enemies, on battlefields the world over. For terrorist groups such as the Islamic State and al-Qaeda, the possession of the black rifle in their fighters’ hands and propaganda videos indicates position, seniority and a sort of material mastery over their accidental American suppliers.

Using a .223 cartridge (meaning the bullet diameter is about .223 inches), the AR-15 has a projectile velocity, depending on the type of ammunition, that can be upward of 3, 200 feet per second and is accurate up to about 500 yards. Known to the masses as an “AR-15 assault rifle” and frequently misnamed as a “machine gun, ” the AR-15 is sold mostly as a semiautomatic weapon only, meaning one pull of the trigger equates to one bullet leaving the barrel. An assault rifle, by definition, means that the weapon is fully automatic.

Fully automatic AR-15s are available in the United States, but they require extensive paperwork from the Bureau of Alcohol, Tobacco, Firearms and Explosives and are much more expensive than their single-fire counterparts. That being said, certain modifications to the weapon’s trigger assembly can enable some semiautomatic AR-15s for fully automatic fire. These devices, known as “auto sears, ” without proper authorization from the ATF, are illegal. It is unclear whether Omar Mateen, the man identified as the shooter at the Orlando nightclub, had a full auto variant AR-15. Even a semiautomatic AR-15 can fire a large number of rounds in a very short amount of time, about as fast as a shooter can aim and pull the trigger.

Typically, an AR-15 is loaded with a 30-round magazine. Although some states regulate the size and availability of magazines — for instance, California allows only 10-round magazines that cannot be removed from the rifle — Florida does not. The magazines are easy to carry, and in the hands of a trained shooter, easy to manipulate and reload. Larger magazines that can carry 75 to 100 rounds are commercially available, but they are unwieldy and sometimes prone to jamming. It is unclear what exactly Mateen was carrying.

The AR-15’s combination of portability, relatively light weight (about 8 to 9 pounds loaded) and customization options make it attractive for both close- and medium- to long-range engagements and the preferred weapon used to kill the enemies of the United States. The military variants are customized and used by every branch of the military for myriad missions, including clearing oil rigs and patrolling the large expanses of Afghanistan.

Although the AR-15 has been standard issue for American service members for decades, the weapon’s ascension to a nationwide staple is a bit of a mystery. Conceived by a company started in a Hollywood garage and solicited by an unlikely trio made up of an aeronautical engineer, an arms salesman and a Marine, the AR-15, (AR standing for ArmaLite Rifle) was born in the late 1950s and came of age during the Vietnam War as an answer to Mikhail Kalashnikov’s AK-47.

Chronicled extensively in New York Times reporter C.J. Chivers’s book “The GUN, ” the AR-15, and eventually the M-16, was introduced as a replacement for the U.S. military’s M-14, a long large-caliber rifle based on an older World War II design. A small number of AR-15s were first bought by the Air Force in 1962 after a bit of salesmanship by Colt Firearms executives (Colt bought ArmaLite in 1959), that involved a pair of exploding watermelons and a general who disliked the M-14. With the Air Force’s initial purchase, the AR-15 entered the U.S. military’s arms procurement pipeline.

After a series of tests and eventual adoption by other branches of the U.S. military, the AR-15, now the M-16, entered the jungles and rice paddies of Vietnam as a malfunction-prone mess. The weapon, after failing in combat time and time again, eventually prompted a 1967 congressional inquiry and a Marine Corps investigation after a Marine officer criticized the weapon in a widely read letter published in The Washington Post, Chivers said.

The M-16 and its civilian counterpart bear only an external resemblance to its Vietnam-era ancestors and are considered mostly reliable, if properly maintained, by today’s standards.

As the U.S. military’s standard-issue rifle evolved, so did the country’s relationship with it. Following the expiration of President Bill Clinton’s 1994 federal assault weapons ban in 2004, the weapon entered its heyday with the American public. Currently, the weapon’s price oscillates between about $800 and $1, 800, given the manufacturer brand. Although commonly sold in standard matte black, some rifles are available in pink for those who are intimidated by its militaristic appearance.