M66 Ring Mount Components
The M66 Ring Mount was developed late in World War II to add antiaircraft armament to ground vehicles such as troop and weapons carriers. It was also installed on cargo convoy vehicles up to five-ton 6x6 trucks. The M66 allows a standing gunner to rotate the machine gun through 360 degrees horizontally while simultaneously permitting sufficient vertical elevation to defend against aircraft overhead. Originally designed to accommodate the belt-fed 50-caliber Browning M2HB machine gun, the M66 mount was later updated to mount the 7.62 mm M60.
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Rotating Rings
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The ring assembly consists of two cast-steel concentric rings, a machine-gun mount and a pintle attachment point. The outer ring is fixed to the frame of the mount while the inner ring rotates freely through 360 degrees on steel roller bearings lubricated with standard issue Automotive and Artillery (GAA) grease. The ring mount uses a clamping brake mechanism installed on the inner ring adjacent to the gunner's position to lock the ring during firing. The machine gun and ring mount are permitted to travel in an unlocked, ready-loaded condition with the vehicle in motion only if the gunner is positioned in the ring mount at all times to control operation of the gun.
Mounts and Ammunition
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A pintle mounted to the inner ring allows rapid attachment/removal of the mounts for the M60 or M2HB machine guns. The mounts incorporate a spring-loaded equilibrilator that counters the weight of the machine gun to allow balanced return to a neutral horizontal position when the gunner is finished firing. Both the pintle and machine-gun mount have removable travel stop pins to immobilize the gun horizontally and vertically while the vehicle is in motion. Twin ammunition boxes mounted to the ring-mount support feed cartridge belts into the machine gun. A canvas cartridge deflector catches spent cartridges as they are ejected from the machine gun to protect occupants in the close confines of the vehicle from being struck or burned by hot brass shells.
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Gunner Operation
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The gunner is positioned in the center of the ring in a standing posture. The gunner's back is supported by a backrest mounted to the inner ring so gunner, ring and machine gun rotate as a single unit to rapidly track strafing aircraft as they move across the gunner's firing horizon. After travel stops are removed from the machine-gun mount and pintle, the gunner loads the ammunition belt into the machine gun. The pintle clamping screw is loosened enough to allow the gun to swivel in the pintle. When a target appears, the gunner releases the ring brake assembly by pivoting the handle clockwise into the "Unlocked" position. Leaning his weight again the backrest, the gunner rotates the entire ring assembly including the gun to the desired position for firing, then pivots the brake handle into the "Locked" position to secure the ring and prevent incidental rotation caused by recoil.
Structure
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The ring-mount assembly weighs more than 300 pounds. It is supported by four steel tube supports. The rear supports are commonly mounted to one of the transverse beams in the vehicle frame. The front supports are mounted to the cab, floor or body of the vehicle. In order to support the recoil forces of the machine gun, standard cargo trucks may require the prior addition of reinforced mounting points for the attachment of the front supports.
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