Ordnance Notes -- by Bob Stoner GMCM (SW) Ret.

GAU-2B/A 7.62mm Mini-gun

An overhead shot of the M134 Mini-gun without its mount. The prominent "horns" are the recoil absorbers for the gun mount, while the tube at the rear is the third attachment point for the gun. The curved object at the top of the picture is a link chute for the stripped links and the feeder/delinker is immediately below it. The square patch near the center of the gun's receiver is the Safety Sector with its quick-release pins. The electric drive motor is located directly below the Safety Sector. Note the barrel clamps around the barrel cluster. Without the barrel clamps, the torque of the gun would warp the barrels and you'd have them looking like a banana peel. The muzzles are not fitted with flash suppressors on this gun. (Photo: US Army)

The GAU-2B/A Mini-gun is a direct descendant of the Civil War-era gun designed by Dr. William Jordan Gatling. Gatling devised the first rotating barrel, manually-operated machine gun. Unfortunately, the Union Chief of Ordnance thought that Gatling's design was some nefarious plot to give the Confederate's victory in the war! A few Gatlings did see service during the war, but their record was spotty.

Gatling sold his guns worldwide after the Civil War. He continued the development of the gun to increase its reliability. However, the invention of the automatic machine gun by Dr. Hiram Maxim in the 1880s doomed the Gatling to obsolescence. Gatling applied an electric motor to his design in the 1890s and achieved a 3,000 round per minute rate of fire from the gun, but the Maxim was on its way to dominance in the first of the Twentieth
Century's Great Wars.

Fast forward to the 1950s. The .50 caliber Brownings on F-86 Sabre jets shot down cannon-armed Russian-built MiG-15 fighters at rates of over 10 to 1. But, the fliers wanted the punch of cannon because the .50 projectile was of limited internal capacity and the 20mm shell seemed to offer the best way to down an opponent. The problem was that jet-to-jet engagement times were very brief. Efforts to raise the cyclic rates of the M3, M24, and M39 guns then in service or projected for service seemed to have reached a plateau of development at approximately 1,200 rounds per minute.

Someone in Army Ordnance remembered Gatling's experiments with the electric motor. Two .45-70 Gatlings were obtained and fitted with electric motors as a proof-of-concept of the design. Despite the jury-rigged nature of the hybrid, the guns did indeed fire at rates of 3,000 rounds per minute or more. Ordnance was astonished. By reverting to the rotary barrel concept they had solved the problems of high cyclic rate, overheating, and barrel wear. Electric drive also solved the problem of jams caused by faulty ammunition and made high reliability possible.

General Electric was given the go ahead to produce a 20mm rotary-barreled aircraft cannon for the "Century Series" of fighters beginning with the F-104. Hydraulic drive was selected for the first 20mm "Vulcan" guns because the electric motors tended to start too quickly and pull the ammunition links apart. Hydraulic drive guns were limited to 4,000 rounds per minute of linked ammunition. Fired brass and links were not dumped overboard as was the case with World War 2 fighters because the higher speed of the jets made the possibility of FOD (foreign object damage) to the aircraft too great. (Several early jets had shot themselves down this way.) Instead, the brass and links were collected in bins that were dumped when the jet landed and rearmed.

The 20mm "Vulcan" came into its own with the adoption of the link-less feed concept and electric drive. The 20mm ammunition was fed to the ammunition drum in links. A delinker was fitted to one end of the drum. The interior of the drum contained a screw-shaped feeder. A linker was fitted to the opposite end of the drum. The linker was connected to the delinker by link chutes. New ammunition was fed to the delinker where its links were removed and transported to the linker; loose rounds of ammunition were fed into the bottom of the drum. Fired cases exited the top of the drum where they were relinked with the links that had originally carried the live ammunition.

Once the drum was loaded, the linker and delinker were removed and the feed chutes were connected to the gun. One feed chute contained live ammunition from the top of the drum for the gun's feeder, while the second feed chute allowed empty cases to return to the bottom end of the drum for storage.

When the pilot squeezed his trigger, an electric motor started the gun barrels spinning. The motor also drove internal gears which powered the feeder on the gun. An electric motor also powered the screw feeder in the drum. Rounds were admitted to the gun feeder by a solenoid-operated feed pawl. If the gun was mounted in the aircraft with the top barrel at 12 o'clock position, the feeder received the incoming ammunition at 4 o'clock, transported the round counter clockwise to the 12 o'clock where it was fired, and transported to the 6 o'clock position where it exited the gun feeder. (NOTE: clock positions are as seen from the rear of the gun.)

When the pilot released the trigger, the feed pawl blocked the flow of live rounds to the gun. The barrel inertia caused the barrels to continue spinning. This inertia ensured that any unfired rounds in the gun were fired and the empties were cleared from the gun. As the empties were cleared from the gun, the individual bolts moved to the rear of the gun into what is called the "clearing path."

When the pilot squeezed the trigger again, all the electrical drives and solenoids were energized. The bolts moved forward into the "feed path" and the cycle began again. The maximum rate of fire for the 20mm "Vulcan" M61 gun is an astounding 6,000 rounds per minute (100 shots a second)! Cyclic rate is entirely dependent upon the speed of the electric motors. It wasn't long before GE began experimenting with the Gatling principle in other calibers and numbers of barrel combinations. Ideas included gun pods which were self-contained gun pods and modified versions of the M61 for helicopters. The M61 was modified for use in gun pods to arm fighters that were solely missile armed. (This was a stop gap until an internal gun could be fitted. Guns had been omitted from fighter designs of the mid-1950s in favor of missiles. Unfortunately, the Korean war-era MiG-17 (with guns) could out-match a Mach 2 F-4 "Phantom" with only missile armament.) The gun pod carried the gun, ammunition, controller, feeders, and drive motor for the pod. Some pods used the aircraft electrical system, while other pods were driven by the aircraft's slip stream.

The M197 20mm gun was an M61 cut down to three barrels for mounting in pods on AH-1G "Cobra" gunships or in the chin turret of the helo. A miniaturized version of the M61 was developed in 7.62mm NATO. This was the GAU-2B/A (also known as the M134 or Mini-gun). Minis were originally mounted in pods. They were hung on helicopters and attack aircraft for attacking infantry and soft targets with high volumes of fire. (One Mini-gun could put a 7.62mm bullet into every square foot of a football field.) GE also marketed prototypes of their electrified Gatling in .50 Browning machinegun caliber and 5.56mm M16 rifle caliber; however, the designs did not go into service. The problem with the Mini-gun in a pod was it was limited by the amount of ammunition the pod could carry. Designs soon appeared that turned the Mini-gun into a turret-mounted gun (AH-1G "Cobra"), into a flexible gun mounted in the doors of UH-1 "Hueys" and HH-3 "Jolly Green Giants", and twin side mounts that replaced the four M60C guns on the UH-1B/C gunship helicopters.

An interior shot of a HA(L)-3 “Seawolf” UH-1B gunship of Detachment ONE based at SEA FLOAT/SOLID ANCHOR. This is a home-made Mini-gun installation; compare the improvised gun mount compared to the production types shown in later photos. The M60 in the foreground is a flexible gun that was extensively modified by the door gunners of HA(L)-3. Note the lack of butt stock and forearm, protruding mechanical buffer to boost the rate of fire, a second offset pistol grip attached to the gun with hose clamps, sights removed, and a stripped barrel. (Photo: www.seawolves.org)
Another shot of the field-modification Mini-gun mount on a HA(L)-3 UH-1B gunship. (Photo: www.seawolves.org)

The main differences and similarities between the Mini-gun and its M61 parent are these:

  • The Mini-gun uses linked 7.62mm NATO ammunition; the M61 uses link-less 20mm.

  • The Mini-gun dumps its links and brass overboard; the M61 does not.

  • The Mini-gun dumps 8 to 12 live rounds at cease fire as the barrels spin down; the M61 fires all its live 20mm at spin down.

  • The Mini-gun uses a feeder-delinker to strip linked ammunition from its links; the M61 uses a link-less feeder.

  • The Mini-gun uses flash suppressors on the ends of the barrels; the M61 does not.

  • Both Mini-gun and M61 use timing buttons on their barrel clusters and feeder-delinker/feeders to time them.

GMG3 Paul Cagle fires the Mini-gun on the HSSC of Project ZULU at My Tho in 1967. The spent case and link bag is below the gun; the controller is the box in the foreground; the ammunition box is to the right background. (Photo: Paul Cagle)

The guns fitted to the Heavy SEAL Support Craft (HSSC) and Medium SEAL Support Craft (MSSC) were the flexible types used by the helicopter door gunners. Our adaptations used a 3,500 round ready-service box below the gun. (In actual use we found we could cram in an extra 300 rounds for 3,800.) There was a booster motor attached to the top of the box and a flexible feed chute connected the booster motor to the feeder-delinker of the gun. The guns had a link and brass catcher fitted below the gun. The gun had an electronic motor controller which varied the speed of the drive motor to produce a cyclic rate of either 2,000 or 4,000 rounds per minute. Firing was done by setting the ARMING SWITCH to ON (this energized the two pistol grip-mounted triggers). The left trigger started the gun firing at 2,000 rounds per minute rate (and held down). The right trigger increased the motor speed to get 4,000 rounds per minute. Cease fire was done by releasing both triggers. Shutdown was by setting the ARMING SWITCH to OFF (closing the cover). Power for the motors and motor controller came from a direct connection to the boat batteries.

 

Even with flash suppressors, the muzzle blast from the Mini-gun was impressive in daylight. This is the Mini-gun of the HA(L)-3 UH-1B gunship firing against VC/NVA targets near SEA FLOAT/SOLID ANCHOR.  (Photo: www.seawolves.org)
An interior shot of the Mini-gun as fitted to the Medium SEAL Support Craft of MST-2 Detachment ALPHA. The basket beneath the gun cradle collects the links and brass. Note the double triggers: the left gives 2,000 rounds per minute; the right (after the left is squeezed) gives 4,000 rounds per minute. The ammunition feed chute is below the gunner’s hand and attaches to a feed booster atop the ammunition box. The cable hanging from the gun is connected to the electric drive motor and to the motor controller. (Photo: Gary Hunt)

The M134 gun used by the AH-1G had a slightly different operation than the guns of the HSSC and MSSC. The feed solenoid by the feeder-delinker was always had 24 volts applied to it -- and this made it very dangerous to the uninformed.

When you fed ammo into the feeder-linker on the HSSC or MSSC Mini-gun, you brought the ammo and link to the mouth of the feeder-delinker and turned the barrel cluster by hand. The internal gearing would turn the feeder-delinker. You continued turning until a link fell out. This indicated the gun was loaded and ready to feed. However, the gun would not permit the live rounds to go from the feeder-delinker to the gun unless the feed solenoid was powered. When the solenoid engaged, it tripped the feed pawl which allowed the ammunition to transit to the gun for firing.

The feed solenoid on the AH-1G was always HOT. Therefore, when you turned the barrels to load the feeder-delinker, the feed pawl allowed ammo to feed to the gun. If you continued to turn the barrel cluster, the gun would fire! Since the Army couldn't always trust the ground crews to disconnect the feed solenoid connector when they rearmed, they devised a bullet trap to put over the end of the barrel cluster whenever live ammunition was fed to the feeder-delinker. In this way, some poor bystander wasn't shot during reloading.

Operation and characteristics of the Mini-gun are as follows (looking from the rear of the gun):

  • Ammunition is fed to the gun at 4 o'clock into the feeder-delinker.

  • The feeder-delinker separates the link from the round by a link stripper. The round is carried to the gun by a star-wheel.

  • When the feed solenoid engages, ammunition is passed into the gun in front of the breech bolt.

  • There is a breech bolt for each barrel (for a total of six). The breech bolt rollers move back and forth in an oval cam track.

  • The breech bolt cam track accomplishes feeding, chambering, locking, firing, unlocking, extraction, and ejection.

  • Locking, firing, and unlocking are accomplished by a detachable part of the track called the Safety Sector.

  • The Safety Sector is held by two quick release pins. If removed, the gun cannot fire because the firing pin doesn't work.

  • All the innards of the bolts assemblies, cam track and bolt tracks are lubricated with Teflon semi-fluid lube oil, LSA-T.

  • Each barrel fires at the 12 o'clock position as it comes into alignment by the Safety Sector.

  • As the barrels continue to rotate (counter clockwise), the bolts extract the round from the barrel.

  • As the bolts pass the ejection port (at 6 o'clock), the fired case falls out.

  • Stripped links fall out of the feeder-delinker as the barrel cluster turns.

  • At cease fire, the feed solenoid blocks feeding of rounds from the feeder-delinker to the bolt.

  • As the barrel cluster spins down, 8 to 12 stripped links and live rounds cycle out of the feeder-delinker.

  • The spin-down feature of the gun fires any rounds in the gun and makes sure it has no live ammo to cook-off between bursts.

  • The Safety Sector, drive motor, and feeder-delinker are attached to the gun by means of quick release pins.

  • Gun timing is done by two buttons; one on the feeder-delinker and one on the barrel cluster. Timing is done by push them either one by one or together.

  • The flexible gun mounts in an A-frame with shock mountings to damp recoil and barrel torque.

Since the end of the Vietnam War, the Mini-gun went out of production at General Electric. Mike Dillon of Dillon Aero bought the rights to the Mini-gun. Dillon made production improvements to the design and these new guns have been put into service as the GAU-17/A. The new guns have joined the older GAU-2B/A and M134 guns in OPERATION ENDURING FREEDOM in Afghanistan and Iraq.

 

The GAU-17/A Mini-gun mounted on a Marine Corps helicopter in Iraq. This is the product-improved version built for the U.S. military by Dillon Aero. Compare the revised mounting of this version with the following photo of the Mini-gun mounted on the MSSC of Detachment CHARLIE in 1970. (Photo: Dillon Aero)
The Mini-gun mounted on the MSSC of MST-2 Detachment CHARLIE. The LSSC is acting as cover boat for the MSSC. Both Mini-guns used by Detachment CHARLIE had flash suppressors on their muzzles – the “prongs” extending past the forward barrel clamp. At this speed in the confines of a canal, the boat wake on the shoreline could be very dramatic as the photo shows. A sampan is laid across the engine hatches below the gun cradle. The hand-made local craft was used by SEALs for insertions into the smaller canal tributaries that were either too shallow or too narrow for either the LSSC or MSSC. (Photo: Bob Stoner)
 

© 2005 Bob Stoner R3