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M61A1 GAU 4 20-MM Cannon

'Vulcan Cannon'

General Electric
M61A1 GAU 4 20-MM Vulcan Cannon

Designator: M61A1, GAU 4 Vulcan
Name: 20-MM Vulcan CANNON
Type: Gatling-Type Cannon
Manufacturer: General Electric

Weight SUU-16/A pod With 1,200 rounds:1,719 lb (780 kg)
Empty:1,067 lb (484 kg)
SUU-23/A With 1,200 rounds:1,730 lb (785 kg)
Empty:1,078 lb (489 kg)
GAU 4 275 lb (125 kg)
M61A1 gun Standard: 252 lb (114 kg)
Lightweight: 205 lb ( 93 kg)
Dimensions SUU-16/A, 23/A2 pods Length: 16 ft 7 in (5.05 m)
Diameter: 22 in (560 mm)
M61A1 Length: 6 ft 1.4 in (1.86 m)
Diameter: 1 ft 1.5 in (343 mm)
Recoil: 0.25 in (6.4 mm)
Armament bore 6 x 20-mm rifled barrels on a geared rotor mounting
M61A1 is driven by external electrical or hydraulic power
GAU 4 is self-driven by gun exhaust gases
load/fire system bolt on each rotating barrel opens/closes as it follows fixed cam path ammunition feed and storage link-less feed from 1,020 to 1,200 round storage drum
Performance rate of fire typical: 6,000 rpm
maximum: 7,200 rpm
average recoil force 4,000 shots/min: 2,661 lb (1,207 kg)
6,000 shots/min: 3,818 lb (1,732 kg)
muzzle velocity M56 projectile: 3,380 fps (1,030 mps)
PGU-28/B: 3,450 fps (1,052 mps)

DESCRIPTION
The General Electric M61A1 Vulcan is a 6-barrel 20mm cannon used by a wide variety of American combat aircraft.  The 6-barreled GE M61A1 has been the standard internal gun armament on most US combat aircraft for over 30 years. (The self-powered GAU 4 is virtually identical).  It can fire standard 20mmx168 ammunition at up to 7200 rounds per minute. In many applications, the rate of fire is selectable between "Low" 4000rds/min and "High" 6000rds/min. Gun recoil listed above is at the "High" rate of fire.

M61A1 and the F-106
The F-106A surprised everyone by having good maneuverability and showing potential as being an excellent dogfighter. There were some thought to using the F-106 for top cover in Vietnam. Among suggestions were to apply tactical camouflage, fit a clear-view canopy, and add internal cannon armament.

Although the F-106 never actually did serve in Vietnam, the suggestion of the addition of a gun was taken seriously.  The gun was not intended for air-to-air combat against enemy fighters, but was primarily intended to provide extra firepower for a better close-in kill potential against enemy bombers, but it was thought that it might also be useful in attacking bombers flying at low altitude.

In 1968 the Foreign Technology Division of Air Force Systems Command (AFSC) conducted evaluations at Groom Lake called 'Have Doughnut' and 'Have Drill/Have Ferry' to evaluate the effectiveness of existing tactical maneuvers by USAF and USN combat aircraft against the MiG-21 Fishbed (Have Doughnut') and Mig-17 (Have Drill/Have Ferry).  Read more about the Have Drill/Have Ferry Tactical Evaluation in this PDF file Have Drill/Have Ferry Tactical Evaluation (6.3MB PDF).  These excersies included the F-106, results which helped substantiate the need for cannon armament on the Six.  The ADC evaluated the F-106 and found its radar capable of acquisition of the MIG and that radar snap-up attack with all aspect armament could be used to exploit the MiG-21's lack of fire control.  The F-106 could use better acceleration to get beyond MiG-21 speed limits.  A direct result of this evaluation was the ADC recommendation to “expedite fitting the internal gun, redesigned canopy, radar warning and ECM equipment into the F-106”.   ADC 13 missions with the F-106A Delta Dart and following the missions at Groom Lake recommended the expediting fitting the internal gun, redesigned canopy, radar warning and ECM equipment into the F-106.

2 F-106's (believed from the 84th FIS, Castle AFB,CA) with a Mig-17 during projects at Groom Lake 1968.
See more on the Photo Gallery

PROJECT SIX-SHOOTER
In support of a cannon armament program Convair issued a proposal to re-equip the F-106 with an internal cannon, an optical gun-sight, and a clear-view cockpit canopy in a program known as Project Six Shooter in 1972 headed by Bob Archibald.  The internal 20-mm M61A1 rotary cannon with 650 rounds was fitted inside the rear half of the weapons bay, replacing the Genie nuclear-tipped rocket. However, the four AIM-4F/G Super Falcon missiles could still be carried.

The gun system was installed as a package (pod) inside an enclosure mounted inside the rear of the weapons bay between the rear missile launcher rails providing an aerodynamic shield for the portion of the gun protruding below the missile bay and out into the air stream.  Gun-equipped F-106As could be distinguished by a bulged fairing underneath the fuselage which provided clearance for the rotating barrels of the cannon. As part of the program, a new "clear-topped" canopy was tested, which eliminated the metal strip above the pilot's head, markedly improving the cockpit visibility.

The gun installation was first tested 10 February 1969 on F-106A 58-0795 and subsequently on F-106A 59-0092.  A prototype gun-sight was developed at Tyndall AFB.  According to a FY75 ADC History document research by Marty Isham, a total of 75 guns were made by contract at GE and the gun was installed only on those F-106s that had vertical tape instruments.  When firing, the Vulcan was limited to only 4500 rounds per minute, compared to the 6000 rpm available when installed on the F-4E, due to limitations in the hydraulic pump which rotated the weapon. 

This innovation was not provided for the F-106B two-seater.

M61A1 Operation
The M61 operates on the Gatling principle. 6 20-mm barrels are mounted on a geared rotor that is driven by a 20-hp electric motor. As the motor turns the rotor, the cam follower on the bolt of each rotating barrel follows a fixed cam path in the gun housing, opening and closing the bolt once per revolution. Firing only once per revolution reduces each barrel's rate of fire to below that of most single-barrel revolver cannon. GE claims that this continuous rotary motion eliminates the impact loads on gun components and that sharing the thermal duty cycle among 6 barrels "significantly" increases barrel life.  The use of external power eliminates jamming due to a misfired round.

In aircraft with the double-ended, hydraulically driven link-less feed system, rounds stored along longitudinal rails within the drum are moved to the gun end by a helix; the helix is made of Fiber-Reinforced Plastic (FRP) in the F/A-18. A rotating scoop disc assembly transfers the rounds to a rotating retaining ring. The rounds travel partway around the ring to the exit unit, which puts the rounds into the chute that feeds the gun. Empty cases are returned to the drum for storage.

STATUS

Initial operational capability on F-105 Thunderchief in 1958. Built by General Electric Company, Burlington, Vermont. In production and in service with all US armed force branches and with several foreign air forces as well.

VARIANTS
Phalanx CIWS:
Modified M61 for shipboard anti-missile use. Built-in pulse-Doppler J-band fire control radar, and digital computer. See separate entry.

Lightweight M61A1: Lighter weight, has linear link-less feed system, AIM-GUNS fire control software changes that expand the effective gun envelope, and PGU-28/B Semi-Armor-Piercing High Explosive Incendiary (SAPHEI) projectiles.

Sea Vulcan JM-61-MB:  M61 in open mount fitted on some Japanese maritime safety patrol craft.

M35 Armament sub-system: Mounted under the Bell AH-1G Huey Cobra's left sponson, the M35 has the XM-195 gun, which is an M61A1 gun modified with blast deflectors. The system weighs 1,168 lb (530 kg) loaded, 595 lb (270 kg) empty and carries about 950 rounds of ammunition which it fires at 4,500 shots/min.
300 M35 kits purchased beginning in 1968.

USERS/PLATFORMS
The M61 gun is in widespread use by the United States and many other nations on the following aircraft.

USA
A-7 Corsair
F-4 Phantom
F-14 Tomcat
F-15 Eagle
F-16 Fighting Falcon
F-18 Hornet
F-106 Delta Dart
F-111
AH-1G Cobra
Brazil/Italy
AMX
Aeritalia F-104S Starfighter
Japan
F-1
T-2
Other aircraft can accept M61 in the SUU-16 or SUU-23 externally mounted
As part of the Phalanx Close-in Weapon System (CIWS), it is in service on most US Navy ships and many ships of foreign navies.

PERSONAL STORIES & COMMENTS

8/2/2013
The 177th FIG, NJ ANG, the last operational F106 unit until conversion to F16's in 1988 flew the gun. I was in the avionics shop and worked F/L and mockup and saw all the mods to the aircraft at the end of its operational life. Worked the Six for 14 years. Many of my peers worked it for 20 years both active and ANG full time in the technician program.
Ken Zink

8-2-2013
At the 27th at Loring, we knew about the plan (I left end of August 1970), but I personally was not aware that any of the operational squadrons had actually received any Darts with Vulcans.
M. Ross

8/2/2013
Bob Archibald flew the F-106 with the 329th FIS in the 1960's. He was the person that headed the six-shooter project. If you have any questions about the project, I know Bob would be more than happy to answer them.
Darrell Martin

8/3/2013
I was given the privilege of heading up the official Gun checkout program at Tyndall as an IWS instructor. There was a Senior Master Sergeant in the Weapons shop who made all who graduated a plaque with their rank & name bracketed by a 20MM inert round on each side. I am actually looking at mine now, it is over my computer desk. (Two IPS came down form each squadron for their checkout). We used the LCTT (Low cost tow target) towed by a F-101 (there was scoring on the target that read out in the back seat of the 101. I flew the F-106 for 13+ years on active duty and another 7+ in the drone program as the chief pilot and instructor as well as controlling them remotely both manned and unmanned. Amazing A/C, beautiful on the ground and in flight, a dream/blessing to fly. I was blessed with over 20 years in her cockpit and loved every second of it. My special tail numbers were Round eye 504 from Dover (95th FIS) (my first A/C with my name on her) and 590074 from Griffiss AFB, 49th FIS. Took 074 to two William Tells and when she came thru as a Drone I asked our Maintenance chief to put my name on her again. She died a Vikings death at 1.65 mach over 60,000 feet with 3 missiles fired at her simultaneously. I arranged to be controlling her at the time. Sad but better than rusting away in DM. Blessings,
Paul Grignot

8/4/2013
58-0795 was the feasibility installation of the M61 gun in the Six. (a Tyndall A/C at the time, some of the "original" Selfridge troops may remember it as it came from Loring to the 94th during the replacement of F-86s in 1960 and was later sent to Tyndall) It was a hard mount- that aircraft had no capability to carry the MB-1. By 1972, the decision had been made to equip (at least the Vertical Instrument "A" models) with the interchangeable design and the 4750 Test Sq, Air Force Academy profs, and JLM (Some MIT professors) worked on developing the "hot line" gunsight using 795. [795 had basically a "ring and post" sight initially installed] Lt Col Albert Pryess was the lead professor from the AFA and had postulated the C-130 gun sight earlier. Maj John Mantei did the lion's share of the test flying during the development process. (I believe he wrote a paper about this for the Avionics(?) Laboratory or somewhere in AF history writings). This was done through summer and fall of 1972. The gunsight computations were not integrated into the "normal" computer program during the development. Fiberglass Aerial Targets (FIGAT) and other tow targets using F-101s were also investigated and used during the testing. By late 1973 the prototype "changeable gun to MB-1" aircraft (59-0092) arrived at Tyndall and the Power Upgrade Program proto proof testing was completed using this aircraft. At this time Sandia Labs brought an instrumented rocket to fire and re-certify the configuration for Nuclear Weapon carriage since the blast shield and armament bay had physical changes. Some of you may have the video "Fantastically Fabulous F-106" which shows a gun installation being performed on 092 in 1974.
Mike Trefethen

Aug 10, 2013
I don't know how many 6's were fitted with the gun, but not all were. Tom Wotring would know as he flew most of them when they were converted to targets. However, I don't think Tom is available now. Cheers
Dick Merkle

Aug 10, 2013
I was the IWS F-106 Flight commander and we were responding to the request from Gen Catton and Gen. Moore as to the capability of the F-106 to provide the Thud close escort during the Rolling Thunder battle campaign. The gun, the canopy, in flight refueling came from this program to turn the Dart into an air to air kick ass machine. This included the then classified programs of HAVE DRILL/HAVE DOUGHNUT. We canceled an IWS student class and initiated COLLEGE DART TO develop tactics against the MIG-21 Atoll/gun threat.. I did take up the gun bird one dark night over the Gulf Coast Test Range and fired a full burst of 600 plus rounds from 45,000 ft. at Mach 1.9+. There was structural damage to the doors aft of the gun do the the high mach temperatures of the skin surface and the M-61 exhaust. But, what dummy in air to air combat is going to be going that speed anyway? I butted head with a bunch of System Command idiots when I tried to get the CUBIC CORP SABOT 20MM rounds installed in the gun. These rounds were molybdenum and depleted uranium. They had plastic around the heavy metal rounds and leaving the muzzle, they fell off and those puppies went out at 6800fps vs. the 3200 fps for the 20 mike mike. Powers to be, I do not remember who, were concerned that the plastic could be ingested into the intakes and cause performance deterioration. They even heaved the equivalent of 1 million rounds into a static J-79 F-4 engine at Eglin and noted some performance degradation. As we all know there is no way that any of that plastic would ever get into the mighty j-75 P-17 P&W power plant due to the location of the M-61 vs. the intakes on the F-106. Even if it did, the engine which digested walnut shells and birds for breakfast would have loved it. FYI, this same type of ammo was approved for the C-130 gun ships in Vietnam and later conflicts. The only thing we really needed was four Aim 9L all aspect Sidewinders on wing stations. Love it. But to answer your question, I have no idea how many of the fleet were modified for the cannon. Come see me at the Udvar-Hazy Smithsonian Air & Space Museum on Dulles airport. The DISCOVERY SPACE SHUTTLE IS THE MAIN ATTRACTION. Cheers,
Fearless Fred Williams, Fighter Pilot Emeritus

8-10-2013
I researched every thing I could find on the Internet about the M61 Sixshooter but none of them mentioned how many F-106A vertical tape aircraft were gun equipped. So I have copied ADC Historian Marty Isham to see if he knows. If he doesn't, we'll have to track down someone who was on the project team and see if they know. Hey, I still remember that fantastic Docent tour you gave the 456th FIS Reunion in 2008 at Udvar-Hazy. That was a memory maker!
Pat Perry

8/13/2013
I was assigned to Flight Test at the AMC Depot at Sac'to in 1971. Occasionally, we would get a gun-modified F-106 through the program. They became more frequent by the time I left Sac'to in 1974. (I seem to recall that early-on there was mention of getting 6 modified birds in each regular squadron first, so they would have a minimal fighter cover package with which to deploy.) In 1974, I went to HQ ADC as a member of the ORI Team, and later until 1978, to HQ ADC Stan Eval. During those years I flew with and was in continuous contact with the six active USAF and six Air National Guard F-106 Squadrons. From the start of that assignment I do not recall any F-106 that was not modified. (I assume the B-models were modified; or I think I would remember, because I gave several instrument, proficiency, and other types of eval check rides in the field during that time.) There is mention of Clear-top Canopy restrictions in the video. Final restrictions must have been removed by the time the birds were modified. I have until now, never, ever heard of any flight restrictions on the canopy. I therefore, flew the standard test flight profile (accelerate to Mach 2.0+, decelerating by climbing to 50,000 ft.) from the very first flight. All Air Defense A-models had been modified. Msg below has mention of F-106s retro-fitted with the gun. 'Retro-fitted' might be a bit mis-leading. In addition to permanent installation of the Air-to-Air Refueling system, Clear-top Canopy, three Mirrors, and Heads-up Gunsight Display, the bird was modified to accept the mobile GE M-61 20mm Cannon Gun Package. It replaced the AIR-2A rocket which was launched from an ejection rack in the center of the aft bay. The gun system was interchangeable with the AIR-2A rocket ejection rack and occupied the same center aft missile bay location. It could be 'down-loaded' (and replaced by the AIR-2A rack) or 'up-loaded' according to the mission. And, as far as I know, all ADC (active and guard) aircraft were modified. I do not know if every squadron had the complete number of gun packages for each aircraft. Nor do I know about F-106s that belonged to other commands or agencies. Recent pictures I have seen of some of such aircraft indicate they were not modified.
Irv Buck, Parker, CO

August 13, 2013
From my memories at the Tyndall 475th Test Sqdn and at the 49th, the gun was a huge upgrade, pilots loved it and it brought the 106 far closer to be formidable than any of its AIM-4 missiles. The 'wet noodle' gun sight worked!! --- And was improved by guys like Stu Cranston and others at the test sqdn. We all loved flying the SIX; too bad its armament always trailed behind the times -- and the targets!! Cheers
John Fuhrmann

8-11-2013
M-61 kits were sent to the 318th, 87th and 49th FISs and some to the ADWC. It wasn't a permanent attachment and I don't know how many were sent to the guard, I know the NJ guys got some kits. They loved it!! I called Dick Stultz and he's working the question also. He said that Manly Ray and Randy Smith mite be able to supply some answers. Hope I've helped...pls keep me in the loop on this one. Cheers
Marty Isham

8-11-2013
per Doug Barbier
ALL Verticle Tape 'A' 6's were fitted for the gun back in the early 70s. The guns, made by GE in Essex VT, were put in storage for almost 10 yrs and weren't flown by the 6 fleet until the pookies were retired, rocket motors deterioration, in the 80s. Any ANG 6 unit with Vert tape a/c could handle the gun. I will check with Walt Houghton as to how many guns came off the line in VT.
Marty Isham

8-11-2013
Another update from Marty Isham. Marty then called me and had received word from Walt Houghton that he couldn't find out how many guns were produced by GE at Essex Junction, Vermont but was still checking contacts. So it appears that every vertical tape F-106 A (no B Models) were modified to accept the gun mod. That would mean that whenever the vertical tapes began production in 1958 through the 1959 models was gun capable but it does not answer how many guns were made. Before he died, Erv Smalley compiled list of all vertical tape aircraft [as now listed at F-106 Delta Dart 1957 Airframe Lineage]. This page shows that some 1957 model Sixes were converted to vertical tape instruments, but they were all B models (no guns). See notes showing which aircraft were affected, noting that ALL 1958 and 1959 models were vertical tape instruments. So, we know that all 1958-59 A models were gun capable. In Wings of Fame Vol 12: 58-0795 was the first aircraft gun prototype and the second was 59-0092. All round eye Sixes continued with the Genie capability which ended in 1985. The last squadrons to fly gun equipped Sixes were ANG squadrons at Otis AFB, MA and Great Falls Montana. Note: Marty Isham is listed in the WOF magazine credits for his contributions to the F-106 section and widely recognized as the leading authority on ADC. Marty's most recent book, USAF Interceptors - A Military Photo Logbook (1946-1979) was compiled by Marty and David McClaren in 2010. So, the real question is how many gun modules were made for the F-106? Hopefully Walt Houghton can find the answer for us.
P. Perry

8-17-2013
Have a good friend that was in the 49th trying to get a hold of a pilot that was in the sqdn. Jim Vanlaak, might know the answer [How many were made by GE for the F-106 and what aircraft type received them], I hope....
Marty Isham

8-29-2013
Well I got just a bit more info from Major Vanlaak...the 49th had 12 guns with 100 rounds of ammo for each. He doesn't know how many came off the GE line. Thanks to Bill Mahaffey, 101 driver for talking to Maj Vanlaak. Cheers....
Marty

12-12-2013
Marty Isham has found the answers to our F-106 gun questions: How many were made by GE for the F-106 and what aircraft type received them?  On 12-11-13 Marty Isham wrote: "A total of 75 guns were made by contract at GE. They were used in only vertical instrumented a/c.  Found the info in a FY 75 ADC History... Marty". Marty is relentless in his research!
Pat P.

     



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