The SKS is a Soviet semi-automatic rifle

Semi-automatic firearm

A semi-automatic, or self-loading firearm is a weapon which performs all steps necessary to prepare the weapon to fire again after firing—assuming cartridges remain in the weapon's feed device or magazine...

 chambered for the 7.62x39mm round, designed in 1943 by Sergei Gavrilovich Simonov

Sergei Gavrilovich Simonov

Sergei Gavrilovich Simonov was a Soviet weapons designer; he is one of the fathers of the modern assault rifle....

. SKS-45 is an acronym for Samozaryadnyj Karabin sistemy Simonova, 1945 Simonov system, 1945), or SKS 45. The Sks is a scaled down version of the PTRS-41 anti-tank rifle also designed by Simonov. The Soviets rather quickly phased the SKS carbine out of front-line service, replacing it with the AK-47

AK-47

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...

, but it remained in second-line service for decades.