The Skewer SMG is an unusually-shaped custom LEGO gun. It's not even completely clear how your minifig is supposed to hold it. But this mohawk-ed minifig below is showing you how it's done; and he's double fisting. The unusual shape comes from the gun's name-sake, skewer, protruding from below the barrel of the gun. It's like a new age bayonet. 

Find this SMG and other BrickWarriors sci-fi guns here. The minifig with a Mohawk is also wearing a Bandana. The unlucky fellow with the Skewer SMG to the back of his head is holding a Resistance Sniper.