This is a UH-1C Shark gunship with the XM-21 "minigun" armament subsystem.
The following description is taken predominently from a book by the former Vietnam helicopter crew chief Pete Harlem, entitled "Crewchief 1: The UH-1C Huey" copywrite 1985 by the Cobra Company, 10225 Montego Bay Drive, Cutler Ridge, FL 33189. You can also e-mail Pete at: Cobra6@aol.com
XM-21 Subsystem: This armament subsystem was mounted on the XM-156 Multiarmament Mount that was used on all aircraft with the seven or nineteen shot rocket pods, and was part of both the XM-16 (quad M-60CA1 7.62mm machine guns) and the XM-21 (twin minigun) subsystems.
Included with the XM-156 was the XM-60 Reflex Sight used by the pilot to fire the rockets. The XM-60 was mounted on the windshield frame above the right seat. It could be stowed by pivoting to the left.
Each turret of the XM-21 subsystem carried a single M-134 7.62mm Minigun, which was fed ammo by a single flexible chute that led from the ammo box mounted against the aft bulkhead in the cabin and passed through holes in the floor of the cabin and fuselage in order to clear the sliding door. Each minigun was supplied by two rows of ammo boxes (six total) via a continuous belt of linked ammo. The belt routed from one row of boxes to the other via a cross-over chute and ammo drive motor assembly that attached to the end of the boxes. The right XM-21 minigun draws its ammo from the aft two rows of interconnected ammo boxes (see photo below) in the ammo system. Note the crossover feed assembly and cartridge drive motor attached to the rear box rows. Each gun was driven by the attached electric motor that spun the six barrels and provided a limited ability to pull ammo from the chute. The miniguns were aimed with a flexible reflex sighting station that, when not stowed, hung down from the roof above the left pilot's seat. This sight controlled the miniguns and depressed and flexed them as needed. This XM-21 with seven-shot pods was the most common weapon setup used on the UH-1C in Vietnam.
(Top three photos by Jim McDaniel, 1967. Lower-right photo by J.C. Pennington, 1969. Lower-left ammo tray photo from Pete Harlan's book.)