That is a replica of the XM25 25mm grenade launcher. It has now been deployed and is called the M25 Counter Defilade Target Engagement (CDTE) system - which probebly isn't what the soldiers are calling it. It fires a 25mm grenade with an electronic fuze that actually counts the revolutions (spin) of the projectile after it is fired. When the set count is reached, the warhead detonates. That huge scope on top of the weapon is a laser rangefinder that determines the range to the target and then a computer sets the fuze in the chamber.