This photo is of the right door gunner on a UH-1C "Falcon" gunship from the 155th AHC working out as the helicopter flies from left to right across your monitor. The gun is what we called a "free" M-60 machine gun, meaning it was hand-held by the door gunner who was sitting on a canvas seat in the cargo area, firing out the side of the Huey. The gun was the basic M-60 machine gun with some modifications, such as the having the stock and bipod removed and some other minor mods. It fired 600 rounds per minute. It was "almost routine" for the door gunner to lean way outside the helicopter firing his weapon, and some would even step out onto the skid so they could fire UNDER the aircraft during breaks following a gun run (when the helicopter was most vulnerable to enemy ground fire). These M-60s also fired a mix of tracers and ball ammo, although some gunners fired pure tracers. In this photo, it appears you can see the muzzle flashes of the doorgunner's gun, and his tracers. The rotating beacon that is also captured during this time-lapse photo is probably the second gunship (the flight track of the second ship does not appear to match the flight track of the gunship doing the firing).
(Photo property of Mark Stuart)