The AR-18 is an assault rifle which fires 5.56 mm NATO ammunition. Designed by Eugene Stoner at ArmaLite in the late 1950s as a low-cost alternative to their own AR-15, their interest in the design waned when the AR-15 was selected for production by the US Army as the M16. Small numbers were later produced as the AR-180 for the civilian market between 1969 and 1972, and several licensed versions of this model were also produced. Interestingly the basic mechanism of the AR-18 has proved to be far more popular than that of the AR-15, and has been used as the basis for many modern designs.

In 1955 the US Army was holding trials for a "battle rifle" to replace their outdated M1 Garands with a new weapon able to fire in fully automatic mode. The Germans had already discovered the utility of an "intermediate" round for this role that was smaller and resulted in less recoil, but the US maintained a number of "big gun" adherents and selected a very powerful round now known as 7.62mm NATO. Springfield Armory and Fabrique Nationale both entered designs based on fairly conventional engineering.

Stoner's entry for ArmaLite came too late for official recognition, but his AR-10 was a dramatic improvement on conventional thinking. Using a number of advanced features and materials, the AR-10 was almost two pounds lighter than either of the existing designs, and offered much better control in full-auto due to its unique "in-line" design. Many in the Army considered to far too radical to be considered seriously, and when a barrel failed in testing it was pretty much rejected.

Only two years later the Army was once again looking for a new rifle, this time based on a much smaller .22 caliber round. This time ArmaLite was invited to compete right from the start, and responded with a smaller version of the AR-10 as the AR-15. In some ways the AR-15 was less advanced than the AR-10, they did not use the composite barrel or titanium parts for instance, but nevertheless there was never any serious competition. The Winchester Arms offering was a remake of the Garand, and Springfield Armory never even entered due to internal debates over the usefulness of a small round. However the entire .22 caliber project was overshadowed by powerful elements of the Army command, who considered any lightweight weapon as puny and looked for any excuse to not put them into service.

In order to help the Army make the desision, ArmaLite started the design of two lower-cost rifles. The AR-16 was a 7.62mm counterpart to the AR-10, and the AR-18 was the 5.56mm counterparty to the AR-15. Key to the cost reduction was the use of a stamped-metal receiver area, as opposed to the AR-10/15's milled aluminum one. In addition a simpler firing system with an operating rod was selected in place of the lightweight gas tube used in the earlier designs, although this was intended to address complaints about reliability and fouling, as opposed to cost. The bolt rode on two rods containing the operating springs, instead of riding directly on the receiver, which also allowed the gun to be somewhat more compact and allowed for a folding stock, as opposed to the AR-10/15 where the buffer springs were in the stock and had to be in-line with the barrel.

Generally the new designs were much more conventional that previous ArmaLite designs, and suffered for it in weight. On the upside they were less expensive and much more resistant to fouling.

The AR-16 was never even studied past the prototype stage, but the AR-18 was put into testing as an alternative to the AR-15. However several parts of the operating mechanism failed in testing, and would have to be redesigned in order to be stronger. When the AR-15 was selected for production, work on the AR-18 was basically dropped and the flaws remained.

A semi-automatic version known as the AR-180 was later produced for the civilian market between 1969 and 1972. Howa, of Japan, continued production of the AR-180 from 1972 until 1974, and Sterling in the UK produced them from 1976 to the mid-1980s. Many of the Sterling models found their way into the hands of the IRA, where it was known as the "widowmaker".

The ArmaLite brand name was recently purchased by what was formerly Eagle Arms, a small US arms manufacturer. An updated model was released in 2001 as the AR-180B, improving on the orignal with a moulded polymer receiver replacing the stamped steel version. This makes the gun much stronger, as well as lighter. A version with a Picatinny rail is likely to be introduced as well.