I've generally run these multi-weapons (HK XM-30 and the Steyr-AUG) as needing the correct skill for each type. That is, Automatics for the AR and SMG modes, Heavy Weapons for LMG and Long Arms for the sniper or shotgun modes. I think the point of these is so you can have a bunch of weapons in one handy package rather than serving as a way to circumvent the need for the right skills to use those weapons.