If they are going to allow the usage of older weapons versus newer weapons, there would need to be some obvious adjustments. Without a doubt newer weapons, disregarding the skill of the user, are superior to that of newer weapons, simply because of newer technology and efficiency. There would have to be a balance between newer and older weapons, otherwise it would be a very bad concept, of which some players would haven an obvious advantage over other players from the very start.
It really depends what time periods the game takes place throughout, though. It is possible that the weapons throughout the times in history that the game goes throughout are similar to one another, especially if contained within the Cold War era.