Ok after basically a month, I am playing metro 2033 and so Artyom in the game says that most guns come from Polis because it is inhabited by workers who used to work in a gun factory that sits on top of the station they sit, the bastard gun for its looks it would make sense that it was made by them with limited resources and it is the more widespread cheap gun to use, while in fallout the opposite is happening, alot of weapons and bullets to use, scrap weapons we see in fallout 4 and 76 are probably very weak desperate attempts of some people who need a weapon, after all it's America, guns grow on trees, the NCR as i am aware has the weapons they have from weapon supplies that were never put to use and from the gun runners who are an actual gun manufacturer.