There is much confusion through the rules about what skill is actually used a couple of types of weapons. The big ones I see are machineguns and grenades. Each of these is mentioned as falling under Heavy Weapons at times, Ranged Weapons at other times. The Gunner archetype is confused with the combat rules and the combat rules are confused with the skills list. (I insist that grenades should fall under Mobility, since using them is actually primarily athletics. That's another story.)
Machineguns are fundamentally the same as most guns. Sure, there's some extra stuff in there, barrel changes, etc. Especially if you're talking about HMGs that must be assembled from 2-3 parts to use. But that still would not make them Heavy Weapons, and the game is very clear about why: it applies a blanket rule that firing heavy weapons at targets smaller than vehicles gets a -2 penalty!
That's all the proof I need that MGs are actually, unquestionably, under Ranged Weapons. Otherwise, you're talking about exceptions to rules on top of exceptions to rules.
I'm sure this will get cleared up, but that right there is the definitive logic if you ask me.