That is the handle which one uses to cycle the action. As you can see in the first pic the soldier has his left hand on the handle. Once he fires the shotgun his left hand will bring back the handle, thus ejecting the spent round. As he pushes the handle forward, the action is brought forward, thus chambering a fresh round out of the mag.
One can see the XM-26 in action in the Jet Li movie "The One" in the scene where the futuristic police are moving the prisoner in the police garage into the armoured bus.