What skill does an underbarrel weapon actually use? For example, the Izom Aphrodite in Gun heaven 2 is a SMG (Automatics) with an underbarrel grenade launcher. Does the GL use Automatics (as the Aphrodite) or Heavy Weapons (per the GL)? What about a gun with a bayonet? Would that use the guns basic skill, or Blades?

The skill used for the underbarrel weapon is defined by its skill, so an underbarrel grenade launcher would use heavy weapons, an underbarrel shotgun would use longarms and a baronet would use blades.