Orlando weapon type cooked up in a Hollywood garage

A newly assembled AR-15 rifle. Photo / AP

The type of rifle used to kill 50 people and injure 53 at an Orlando nightclub - in the worst mass shooting in US 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 rifle and carbines. It's favoured 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, California, Aurora, Colorado, and Newtown, Connecticut.

Its design and likeness appears in video games, movies, television shows and toy stores. Military variants of the weapon have been shipped overseas en masse, arming US allies, and sometimes enemies, on battlefields the world over.

For terrorist groups such as Isis (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.

Chambered in a .223 cartridge (meaning the bullet diameter is about .5cm), the AR-15's projectile velocity, depending on the type of ammunition, can be upward of 975m per second and is accurate up to about 460m.

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 and Firearms and are exponentially 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 authorisation 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 semi-automatic 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 US states relegate 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, however they are unwieldy and sometimes prone to jamming. It is unclear what exactly Mateen was carrying.

The AR-15s' combination of portability, relatively light weight (about 3.5kg loaded) and customisation options make it attractive for both close- and medium- to long-range engagements and is the preferred weapon used to kill the enemies of the United States. The military variants are customised 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 member 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 CJ Chivers's book The GUN, the AR-15, and eventually the M-16, was introduced as a replacement to the US military's M-14, a long high-calibre 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 US military's arms procurement pipeline.

After a series of tests and eventual adoption by other branches of the US military, the AR-15, now the M-16, entered the jungle 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 criticised the weapon in a widely read letter published in he Washington Post, Chivers said.

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

As the US 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 US$800 and US$1800 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.

The exact numbers of AR-15s in the United States are hard to come by, as gun manufactures, as Slate points out, do not break down their sales by model.