Ares Alpha has a grenade launcher, but it never says to use the heavy weapons skill. Do you?

The multi-switchable assault rifle (forget name), can convert to a sniper rifle, shotgun, etc but it never says it uses any other skill either.

How does this work?

My group has always ruled unanimously that if a weapon is converted it uses the skill for how it behaves. The skill in using a LMG variant using LMG, etc. I don't think SR4 is clear, but any other way of doing it doesn't seem to make much sense.

Always use the appropriate skill of the weapon in use. Grenade launcher = heavy, sniper rifles = sniper, etc. No free lunches.

The only time that changes is bolting it to a vehicle/drone, then you use gunnery for both, though oddly enough for autosofts, they must be bought like skills, so for the rigger, you use gunnery on an ares alpha, but for the drone to fire it on it's own for better than pilot rating, you have to buy Longarms and Heavy.