The Russian M1910 Maxim was clearly obsolete in the 1930s, and the Soviet military developed a replacement for it – the Degtyarev DS-39. These performed well in testing, but several major flaws were revealed when they started seeing field use (including in the Winter War) and production ceased after just 10,000 were made. The Maxim went back into service, and a crash program was put in place to find a more suitable replacement.
At the time, a young designer named Goryunov was already working on a light machine gun. He and his design team figured that they could pretty easily adapt it to the heavy MG requirements, and the result was the SG-43. Like the DS-39, it was a gun intended for multiple roles, although it did not have the dual rate of fire option of the Degtyarev. It was chambered for the standard 7.62x54R cartridge, using standard Maxim metallic belts. The Goryunov was a simple and economical gun to produce, and used a gas operating system with a side-tilting bolt. The feed system actually shows a lot of similarity to Degtyarev’s post0war RP-46 belt fed adaptation of the DP-27. Anyway, the Goryunov was adopted in 1943 (largely because Degtyarev personally recommended it as better than his own design) and would serve as the standard Soviet heavy machine gun and vehicular machine gun until the 1960s, when it was replaced by the PK.