The SGR-12's design is based on that of the Saiga-12 semi-automatic shotgun. More specifically, the in-game model is the Saiga-12S, complete with 580mm barrel and lightweight folding buttstock.
Like all other in-game shotguns, the SGR-12 is chambered for the use of 12-gauge shells, offering high damage per round. The gun employs 12-round box magazines for its feed system, offering a good amount of sustained fire duration. This gun is a short range killer, just like M1014. Whichever you find first, you can use it.
In real life, the only 12-round magazines available for the Saiga-12 are drum magazines, suggesting that the in-game model uses aftermarket extended box magazines. However, the HUD icon suggests a physically impossible small box configuration.