The M-60 machine guns used by helicopter doorgunners were significantly modified from the basic infantryman's M-60. While both the Dolphin "slick" door gunners and the Shark "gunship" door gunners used these M-60 machine guns firing the standard linked 7.62mm NATO round, the Shark's M-60's were modified differently than the Dolphin's M-60s.
The Shark's M-60s were the basic infantry M-60 with bipod legs and forward sight removed, and with a butt plate cover where the normal shoulder stock was, and it was not mounted to a hard gun mount like the Dolphin's M-60s were. A "bungee cord" attached to the top of the gun (barely visible at the top of the photo above but clearly visible on the photo below) is a stretchable but strong cord that secures the gun to the roof of the helicopter in case the gunner is hit or otherwise drops the gun. This prevents it from falling from the aircraft, yet allows total freedom to fire down, under, and behind the helicopter in flight.
Often the door gunners, wearing a "monkey harness" to prevent them from falling out, would stand outside the helicopter with a foot on either the rocket pods or a landing skid, firing down and under the helicopter, often firing between the skids to protect the vulnerable belly during the break following a gun run. This being done all the while receiving severe wind buffeting by being blasted with 110 to 140 mph winds as they were often firing their weapons outside the protection of the helicopter fuselage.
This break at the end of a gun run was generally the most vulnerable time for a Huey gunship, as the flexible miniguns or 40-mm thumpers could not flex far enough down during the breaks to maintain fire on the enemy positions.
The door gunners did that.
Ammunition for the door guns was stored in either a large metal ammo can at the feet of the gunners or in a wooden "grenade box," and each door gunner carried about 1,000 rounds of ammunition. Being a door gunner on a Shark or any other Huey gunship was not for the faint of heart, and they were all courageous men to be respected. Few really received proper recognition for their courage, dedication, and hard work. (Top photo by Jim McDaniel, November 1967)
In the photo below, a Shark crew chief of a minigun ship is lounging beside his gun waiting for the order to launch. Notice how both M-60's can be seen suspended from the roof of the helicopter by bungee cords. The crew chief here is using a large metal ammo can for his linked ammunition. He has a smoke grenade hanging off the side of the can. With his macinegun unloaded you can easily see the C-rations can (probably Ham & Lima Beans) attached to the side of the M-60. Good detail of the minigun and 7-shot rocket pod. (Photo by Fred Thompson, probably in 1970 or early 71 because of the "nose art" snoopy on the side of the aircraft)