Guns and Rotors

The Dark Magic of Aerial Gunnery

Written By Will Dabbs, MD

2021

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Door guns on tactical aircraft are designed to provide suppressive fire to keep an
enemy occupied during critical operations in and around landing and pickup zones.

What’s the most fun thing you’ve ever done? Might want to keep it to yourself just in case there are sensitive ears about. One of my hands-down coolest experiences was running belt-fed machineguns out of a helicopter.

There are really only two ways you might ever have an opportunity to burn thousands of rounds out the side door of a $26 million helicopter. One is to have a name like Gates or Trump. The other is to be an unwashed redneck kid like me who signed his name, slipped into a military uniform, and got really familiar with Uncle Sam’s helicopters. The end result was fun on an apocalyptic scale.

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The M230 Chain Gun on the AH64 Apache gunship is slaved to the
gunner’s helmet and is capable of firing off-axis while in flight.
Photo: Yari Samurai

The Equipment

All U.S. Army tactical utility helicopters are equipped with door guns. The SpecOps Bad Boys of Task Force 160 rock M134 miniguns but the Big Army used D-model M60s in my day. These airborne versions of the classic Vietnam-era M60 Pig were, in my experience at least, steaming piles of ballistic feces. Thankfully nowadays our boys and girls going downrange wield FN M240H machineguns that are vastly better.

In my first unit we were allotted 24 M60D door guns. These weapons used a common M60 receiver but eschewed the standard rubber-coated forearm. They also employed spade grips and a ring trigger in place of the standard pistol grip of the ground gun. Out of 24 meticulously maintained and perfectly lubricated, we had maybe five that ran well. Those five were a wonder to behold, however.

The M60D uses standard removeable M60 barrel assemblies replete with bipods. Mounting the M60 bipod to the barrel meant our poor unfortunate grunt brethren had to hump an extra bipod with each spare barrel assembly. Up close, it was just sad.

A series of cams unique to each mount limited the gun’s travel so you didn’t shoot up your own rotor system. Ejection control bags caught the empty cases and links so they wouldn’t clutter up the aircraft. With the guns mounted up, ample ammo onboard, and scads of range space cleared just for us, it was time to rock and

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On the left in this U.S. Navy helicopter is an M3 .50-caliber machinegun.
On the right is an electrically powered M134 minigun.

A Wee Spot Of Science

The practical physics governing aerial gunnery out of a moving aircraft is absolutely fascinating. Our M60 machineguns sported barrels rifled with a 1:12" clockwise twist. If my math is correct it means each M80 147-grain FMJ bullet travelling around 2,800 feet per second is spinning at 168,000 rpm. Any object freely subject to gravity accelerates toward the center of the earth at 32.17 feet per second squared. This is why when we shoot our weapons the bullets invariably arc downward until they eventually strike the earth. Factor in a 160-knot (184 mph) crosswind, however, and some of the most amazing things occur.

When firing bullets with a clockwise spin out the right side of a moving aircraft, the airflow across the spinning projectile creates a low-pressure area on the top of the bullet. Driven by the Magnus Effect, this causes each bullet to behave like its own miniature aircraft and fly upward. The practical result is a gunner trying to hit a target on the right side of a moving aircraft actually has to aim underneath it and let the bullets fly upward to impact where desired.

The same effect is manifest in the opposite direction on the left side of the aircraft. The low-pressure area is on the bottom of the bullet and causes the round to plunge precipitously toward the ground sooner than might be the case if fired from a stationary platform. The faster the aircraft the more pronounced the effect. In practice this makes a huge difference.

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Door guns in Will’s day were D-model M60 machineguns — and horribly unreliable.

Early door guns were simply ground guns suspended via
bungee cords from the ceiling of the aircraft.

Real World Applications

Helicopter door guns are by their nature suppressive tools. The intent is not to pick off some bad guy two kilometers distant. The purpose of door guns is to keep the enemy’s head down long enough to get into or out of a landing or pickup zone. Also, from a thermodynamics perspective the rapid airflow across hot barrels does a splendid job of keeping the guns cool even at high round counts.

The M230 30mm chain gun on an AH64 Apache or the M197 20mm Gatling gun on the AH-1S Cobras of my generation will both fire off-axis while the aircraft is in flight. The fire control systems on these gunships compensate for these slipstream effects along with all manner of other variables.

So long as you aren’t getting shot at for real, running machineguns out of combat helicopters is more fun than a barrel of monkeys. However, I can personally attest firing tracers into grassy spaces will inevitably start a truly epic brushfire — a tale for another day.

Photos in this article are in the Public Domain from the Department of Defense.

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Guns and Rotors

The Dark Magic of Aerial Gunnery

The Equipment

A Wee Spot Of Science

Real World Applications

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