The bottom barrel is the pilot laser, the top fires the Plasma bolt, the mag is prob a battery of some sort.
"One of the most effective is the 40-Watt Phased Plasma Rifle, which fires superheated hydrogen compressed to a plasma state. After targeting, a pilot laser in the weapon vaporizes a path through the atmosphere to the target a microsecond before the plasma bolt is produced. The rifle is capable of rate of fire superior to the common resistance weaponry, with minimal risk of overheating or weapon shutdown. The plasma bolt is capable of penetrating nearly all forms of armor protection. Striking an organic target with the super hot plasma bolt will almost certainly result in fatality, including the possibility of explosion or neatly cauterized amputation. A near miss can inflict third degree burns, and 'splash damage' can be as devastating as a traditional grenade blast."
From Sideshow's site.
SAS