[Tyneras]

Ah, I misunderstood. I think cannon would be a better name to prevent confusion.
Thinking on it, I think this is how I'd separate them, based on how perceive minimal overlap between use (and thus skill).

Guns (LAW) Any weapon held and used at head height, generally resting on the shoulder. Not a huge category but distinct from the others.
Guns (Longarm) Held and used in both hands at body to shoulder height.
Guns (Pistol) Held and used in one hand.
Guns (Gunner) Guns that MUST be mounted to be properly used. If you are strong enough to pick it up and fire it effectively (See Arnold's acting career) you are using Longarm. Most bipod weapons are not included, those provide an aim bonus but don't qualify as Gunner.
Guns (Cannon) Direct fire artillery mounted or on wheels. If you can pick it up and use it, it's Longarm. Boundary between Gunner and Cannon is fuzzy, but single shot vs sustained fire is a good metric.

Artillery. Indirect fire weapons. This is an IQ skill.

A lot of Super Bricks would get more mileage out of Longarm than Cannon or Gunner, since they tend to rip such weapons off their mounts and use them like awkward rifles.