The AK-47 is a selective-fire, gas-operated 7. 62⊙39mm assault rifle, first developed in the Soviet Union by Mikhail Kalashnikov. It is officially known as Avtomat Kalashnikova. It is also known as a Kalashnikov, an 'AK', or, in Russian slang, Kalash. Design work on the AK-47 began in the last year of World War II (1945). After the war in 1946, the AK-46 was presented for official military trials. In 1947 the fixed-stock version was introduced into service with select units of the Soviet Army. An early development of the design was the AKS-47 (SSkladnoy or 'folding'), which was equipped with an underfolding metal shoulder stock. In 1949, the AK-47 was officially accepted by the Soviet Armed Forces and used by the majority of the member states of the Warsaw Pact.