“You need a heavy round with a higher volume”

Sea Stallions implement new ramp-mounted weapon system


Cruising over the desert just above 400 feet, a Marine crew chief kneels behind the Gun Ammunition Unit 21 mounted on the ramp of the CH-53D Sea Stallion. The pilots and crew chiefs in the front of the aircraft relay the current target’s location that will be coming into his field of fire within seconds.

GAU-21 Ramp Gun

Marine Heavy Helicopter Squadron 463, Marine Aircraft Group 16 (Reinforced), 3rd Marine Aircraft Wing, is the first CH-53D helicopter squadron to install the GAU-21 .50-caliber machine gun system onto their aircraft.

“The GAU-21 is very similar to the XM-218 .50-caliber, which is the machine gun that is mounted on the doors of the aircraft,” said Cpl. Thomas D. Martinez, crew chief and weapons and tactics instructor, HMH-463. “Most people can’t really tell the difference until they know a little bit about them. It is a new system to the Marine Corps. The XM-218 has been around forever, but the GAU-21 is modified to fire faster and fire slightly farther.”

This next paragraph is sure to resonate with MO readers:

“The 7.62 round used in the M-240G is too small,” said Harquail, a 26-year-old native of Sea Side, New Brunswick, Canada. “The rotor wash from the aircraft affects the rounds’ trajectory. The .50-caliber is a heavier round. You need a heavy round with a higher volume.”

Another view of the GAU-21

Partying with the GAU-21 at Night

Between the muscle required to man the weapon and the buffeting and heat in the rear of the aircraft, the crew chief has his work cut out for him. But they appreciate the weapon’s capability and location.

“We are normally engaged by the enemy from the rear of the aircraft,” said Martinez, a Durango High School graduate. “These weapons are important because our window guns only give us coverage in less than a 180 degree angle in front. The tail gun provides 180 degrees of coverage behind and to the sides of the aircraft.”



  1. I wonder whether a 20mm cannon (something smaller than an M61, perhaps a two-barreled affair) would be too big for an installation like that. I suspect not, but ammunition weight would be a bit limiting. Still, could be pretty devastating.

  2. I certainly hope this weapon system has a verticle stop that prevent the weapon from being raised up very far, as the 53’s have a verticle tail rotor. It certaily would not do well to shoot off your tail rotor.

  3. If the GAU-21 is actualy the FN M-3M then that thing fires at 1100 RPM, not bad for a .50. Some French Cougar helicopters in their version of the SOAR usually mount a single 20mm cannon firing out the left side.