The AK-47 (contraction of Russian: Автомат Калашникова образца 1947 года; Avtomat Kalashnikova obraztsa 1947 goa; "Automatic rifle Kalashnikov model of 1947 year") is a 7.62 mm assault rifle developed in the Soviet Union by Mikhail Kalashnikov in two versions: the fixed stock AK-47 and the AKS-47 (S - So skladnym prikladom) variant equipped with an underfolding metal shoulder stock. Design work on the AK began in 1944. In 1946 the rifle was presented for official military trials, a year later the fixed stock version was introduced into service with select units of the Red Army (the folding stock model was developed later). The AK-47 was officially accepted by the Soviet Armed Forces in 1949. It is also used by the majority of the member states of the former Warsaw Pact. The AK-47 was also used as a basis for the development of countless many other types of individual and crew-served firearms. It was one of the first true assault rifles and, due to its durability and ease of use, remains the most widely used assault rifle in the world. More AK-type rifles have been produced than any other assault rifle.