The last reason may be why the FNX made it in - I'm no gun nut, but I am a casual enthusiast and I'd never heard of it before this game.
It's not that it's a bad idea sacredandprofane, it just really comes down what Gews said - it's likely that the Dev team will choose to stick to guns that either a) fit the setting b ) are popular world wide (the AK and the Remington 870 shotgun are good examples not in game yet) or c) are popular among the Dev team.
They need to focus on all the most common, most suitable and most likely weapons choices first. Unless they're going to model every weapon on the planet, that's not a valid reason to include random guns. Obviously, weapons of any type could be stashed beneath someone's floorboards from WWII days. The devs won't add this weapon because it doesn't fit the setting, there is no demand for it, the ammunition would be exclusive to this one firearm, which is something they don't like, and no, they would not add a random bunch of Blackhawks, AutoMags and Magals to make better use of it. I would not find it 'authentic' to be finding any.
Civilians would not have had them during Soviet times and would not have gone on random M1 Carbine buying sprees after. They wouldn't have been using M1 Carbines prior to the breakup of the Soviet Union and they wouldn't have purchased them after, so the 'army, police and security forces' explanation is a no-go.
