How a ArmaLite AR-18 works

The ArmaLite AR-18 is a gas-operated, selective-fire rifle chambered for 5.56×45mm NATO ammunition. The AR-18 was designed at ArmaLite in California by Arthur Miller, Eugene Stoner, George Sullivan, and Charles Dorchester in 1963 as an alternative to the ArmaLite AR-15 design, which had just been selected by the U.S. military as the M16. While the AR-18 was never adopted as the standard service rifle of any nation, its production licence was sold to companies in Japan and the United Kingdom, and it is said to have influenced many later weapons such as the British SA80, the Singaporean SAR-80 and SR-88, the American Adaptive Combat Rifle, the Belgian FN F2000 and the Heckler and Koch G36] Through the Provisional IRA, which was noted for its use of the rifle, the rifle became known as the "Widowmaker"