Yep... just have those russian and german guns imported to the US then survive 200 years well past nuclear armageddon, at least a springfield is American.
I agree, and this is the main problem with guns like MG42 or a PPSH-41.
I think of the Fallout world is like us in the 50's, except they never got out of that stage (Cold War Tense-y). So I highly doubt you would find a good amount of Soviet weapons spread across the states, and survive the nuclear war + ~200 years. It's such a low chance that it would just be ridiculous if it showed up in-game. I
guess I could understand a museum (but uh, "Weapons Museum" doesn't attract Raiders or anything...
) that had the weapon, but that's still a bit of a stretch.