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Ordnance Notes --
by Bob Stoner GMCM(SW) Ret.
GAU-2B/A 7.62mm Mini-gun
An overhead shot of the
M-134 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. (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 machinegun.
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 machinegun 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/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 M-3, M-24, and M-39 guns then in service or
projected for service seemed to have reached a plateau of development at
approximately 1,200 rounds/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/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/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. 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. The 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" M-61 gun is an astounding 6,000
rounds/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 M-61 for helicopters. The M-61 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 M-197 20mm gun was an M-61 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 M-61 was developed
in 7.62mm NATO. This was the GAU-2B/A (also known as the M-134 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
M-16 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 M-60C guns on the UH-1B/C gunship
helicopters.
The main differences between the Mini-gun and its M-61 parent (other than
caliber) are these:
* The Mini-gun uses linked 7.62mm NATO
ammunition; the M-61 uses link-less 20mm.
* The Mini-gun dumps its links and brass overboard; the M-61 does not.
* The Mini-gun dumps 8 to 12 live rounds at cease fire as the barrels spin
down; the M-61 fires all its live 20mm at spin down.
* The Mini-gun uses a feeder-delinker to strip linked ammunition from its
links; the M-61 uses a link-less feeder.
* The Mini-gun was usually fitted with flash suppressors for the barrels;
the M-61 was not.
* Both Mini-gun and M-61 use timing buttons on their barrel clusters and
feeder-delinker/feeders to time them.
The guns fitted to the HSSC and 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 MSSC 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/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/minute rate (and held down). The right
trigger increased the motor speed to get 4,000 rounds/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.
The M-134 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 of the Mini-gun is 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 (6). 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 is 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; either one by one or together.
* The flexible gun mounts in an A-frame with shock mountings to damp
recoil and barrel torque.
R-1
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