Self-loading rifle that fires an intermediate-power rifle cartridge
Source (Wikipedia): Assault rifle

An assault rifle is a select fire rifle that uses an intermediate-rifle cartridge and a detachable magazine. Assault rifles were first put into mass production and accepted into widespread service during World War II. The first assault rifle to see major usage was the German StG 44, a development of the earlier Mkb 42. While immediately after World War II, NATO countries were equipped with battle rifles, the development of the M16 rifle during the Vietnam War prompted the adoption of assault rifles by the rest of NATO. By the end of the 20th century, assault rifles had become the standard weapon in most of the world’s armies, replacing full-powered rifles and submachine guns in most roles. The two most successful modern assault rifles are the AK-47 and the M16 designs and their derivatives.