I don't actually get why they changed it from 2nd edition where it was just Firearms and Gunnery, and you could concetrate/specialize down from their to be more specific.
It doesn't seem to be much of a real difference to me if a character has Firearms 4 and only ever uses his trusty FN HAR even though he could pick up a pistol just as easily if he needed to, and a character that has Automatics (or Assault Rifles) 4 and no other skills in the group so he would only pick up a pistol if he really needed to.
The real difference: One let's you save your own bacon by dealing with what's available in a way I like to call "realistic enough" - the other says "you lose because you didn't put your points where the plot arbitrarily told you you needed them."
It's a trade-off the player consciously made, though. If a character has a physical track of 11 and a stun track of 10, and then he gets hit with 10 stun is also him going down because of a choice of where to put his points. Same can be said about someone who took low-pain tolerance, causing him to just get one extra -1 during that fire fight, which just happens to cause him to fail something, leading to his doom.
Ultimately, the entire character building process builds down to little choices that could, at some point "arbitrarily" lead to your demise. I'd say your example is even less arbitrary, as many precautions can be taken to make sure you don't end up in that situation.