Well, I think that they could use real life weapons and maybe some upgraded versions of them in the games up until early 70's. At that time the most weapons manufactured still had wooden buttstocks and stuff like that. Even though the world alternates from our timeline it doesn't mean that every single thing has to alternate. But I don't want to see weapons like Glock 17 which is light and mostly made out of plastic, I want them bulky and heavy.
Of course there has been modern weapons in the series before, especially in tactics. But there also were some in Fallout 2 for example H&K G11and P90 and I was kind of dissapointed by those being in there.
Original idea behind new weapons and odd/rare chamber sizes were so that the developers didn't have to worry about realistic damage and effects. I really respect this design decision because it really makes the world unique and I want it to stay that way, it adds more wow factor in to the game world. For once the developers' laziness was a good thing...