This discussion has been had in other threads, but I think some modern weapons are appropriate. Real world "Retro" guns and retrofuturistic Resource Wars guns are obvious fits, but I think modern guns have their place too, if handled carefully. I think the focus should be on rare modern weapons, one-offs that never made it into production, prototypes etc to show that Fallout wasn't just in a freeze over from 1959 to 2077, but also that their version of our "modern era" was still alternate to our own. Having some weapons from the 70s-2000s gives some texture to the setting in that it shows it has a long history, because much like in the real world we don't disintegrate any technology that isn't current, and old stuff is floating about everywhere. Same would apply to the Fallout world.