The underbarrel shotgun is avaiable to currently one Primary weapon whitch of course belongs to the Scar-H.
The attachment itself is rarely seen because the Scar-H is rarely used after it has reached 100 kills. Or from the fact it can not be refilled from an ammo box.
It requires the shotgun ammo perk to load or have ammo for. (Unless you've unloaded a shotgun from anther user.)