Balance is more important than realism and grouping all firearms into the same skill isn't unreasonable, regardless of the specialized skills needed. I mean it's not like firing a rocket launcher, a minigun, and a flamer have a whole lot in common yet they all fit under Big Guns so making an all encompassing guns skill isn't much less reasonable. Having multiple skills is fine as long as they are equally viable, a problem which Fallout has had with many of the skills since the first game and I'd say that all of the weapons skills tended to be more useful than some of the others. It makes even more sense to roll them into a single skill if you make a lot of alternatives to combat and make it play a much smaller/more optional role. This whole discussion assumes that it won't be possible to max out every skill like in Fallout 3, which might be a vain hope.