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Title:	GUN, AUTOMATIC -  U.S. GUN M39A2 20MM SN# 49095
Maker/Manufacturer:	PONTIAC MOTORS DIV. OF GM
Date of Manufacture:	1958
Eminent Figure:
Catalog Number:	SPAR 6577
Measurements:	OL:183.6CM 72 1/3" BL:135.8CM 53 1/2" 179 lbs.

Object Description:

U.S. GUN M39A2 20MM SN# 49095
Manufactured by Pontiac Motors, Pontiac, Mi. - Gas-operated, revolver-type, air-cooled automatic gun which fires electric primed ammunition from a metallic link belt. Ammunition may be fed into the gun from either the right or left side. The weapon is distinguished by a 5-chamber drum which revolves about an axis, parallel to gun bore. Gun fires 20mm cartridges at the index drum position of the 6 o'clock chamber. 9-groove rifling; right hand twist. It is used by the Air Force for fixed type mountings in aircraft. Weapon weighs approximately 179 lbs. Based on Mauser Co. design. Used by Air Force for fixed type mounting in aircraft. Weapon fires the following ammunition: ball, API, HEI, TP and dummy. Muzzle velocity with API and HEI approximately 3,300 fps. Maximum range with HEI 5,750 yards. Weapon currently crated.

Markings:

Weapon transferred to the Museum on 4 August 1961.

Web site photo shows SPAR-6577 in original shipping crate.

Springfield Union, 26 April, 1865 - Letter to the Editor. "The German Gun. Sir: You are to be commended for the April 16 editorial, 'Sticking to the Armory Case.'
The mission of the Springfield Arsenal, it is understood, has always been to establish tolerances and specifications for military small arms and other weapons assigned in the mission established by the chief of ordnance.
To fully accomplish its mission the Arsenal must establish and operate at least a pilot line in order to develop and prove out the tolerances and specifications to be sued by industry in mass production of weapons.
It is questionable, if pilot line production had been left up to industry, whether the World War II armed forces would have had an M-1 rifle which would have stood up under the various conditions encountered by our troops in every corner of the world.
Your editorial points out the 20 millimeter gun to be made in Germany has an exhaust problem when used in an enclosure, as in a weapons carrier.
The most serious problem, would involve logistics. This is maintenance and supplying of spare parts. Because of the metric system of measurement used in the manufacture of the guns, repair units and depots must be provided with tools and equipment made to metric measurements just to be able to service guns bought in Germany. This means increasing the taxpayers' funds for the operation of the Defense Department.
But the worst part would be the confusion arising from the weapons with two standards of measurement.
The one sure way to keep the Springfield Arsenal is to have the congressional appropriations contain mandatory language that certain funds are to be used for the operation of that installation. The Defense Department is obligated to carry out what is contained in public law. - CHARLES L. BULLOCK, Springfield."

HISTORICAL SUMMARY OF ACTIVITIES:
1JUL54 - 31DEC54 - "The Ford Motor Co., under the technical supervision of Springfield Armory, developed a series of modifications to the standard M39 Production Weapon. During this period, approximately 25 modified weapons were fired for endurance and gun reliability. The results of these tests confirmed earlier assumptions that the M39 Gun, modified in accordance with the Ford Motor Co. recommendations, provided a weapon reliability of one gun stoppage per 1,000 rounds. Findings of serial firing trials resulted in the development of component parts which would further enhance gun reliability and endurance characteristics of component parts. Approximately 30 guns, incorporating the latest modifications, were scheduled to be fired during February and March of 1955. Action was initiated to incorporate the latest design changes into production weapons.
The Armour Research Foundation designed and tested a retracting device to reduce the cook-off hazard. Cornell Aeronautical laboratory with Armour Research and North American Aviation Company, scheduled serial firing trials to determine the actual extent of the cook-off condition and to provide remedial measures."
1JAN55 - 30JUN55 - "The Department of Defense cThe Ordnance Corps and Ford Motor Company, in conjunction with the Air Force, came up with engineering improvements to be included in Ford modified (R&D) guns. Twelve of fifteen Ford Motor Company improvements (Phase I) were incorporated with a resultant increase in gun reliability; however, one stoppage per 1,125 rounds fired was still to be obtained. Tests of these modified guns from various agencies bore out that fact.
In March 1955, an Engineering Meeting was held at Springfield Armory in which the M39 Modification Program was reviewed. The stoppage rate was modified at that time to read - not more than one stoppage per three times the complement of ammunition in each gun position.
Springfield Armory reported that the T161E3 link stress member came off at a rate of 1/506 rounds fired. This, of course, was not acceptable and a link investigation followed. George K. Garrett Manufacturing Company, Inc. manufactured a sample lot of T61E3 links in which the stress bridge was spot welded to the link body - these links appear to have corrected the difficulty.
Cook-off studies were being carried out by Cornell Aeronautical Lab., Inc. and Armour Research Foundation designed and built two feeder shaft retracting devices which allow clearing of the drum at cook-off temperatures. On 29 March 1955, a directive was issued which required the Ordnance Corps to fabricate and test a mechanical ADF designed by North American Aviation. Test of this device proved successful and 3550 kits were manufactured by Ford Motor Company to retrofit guns in the field. At about the same time, testing agencies began reporting poor barrel life. Several quick fixes were attempted including chrome plating the barrel face, tapered seals, short seals, various materials butt welded to the barrel face and two-piece seals. Limited tests to date indicated that the two-piece seal increases barrel life two fold or approximately 1,000 rounds per barrel.
Prior to May 1955, additional improvements were incorporated into the weapon (so designated Phase III). This weapon met the overall gun stoppage rate as prescribed by the Air Force. Since then, the Ford Motor Company was awarded a contract study to modify 168 guns in accordance with the Phase III gun configuration."
1JUL55 - 31DEC55 - "The M39 Phase III Weapon Modification Program was approximately 50% complete. Ford Motor Company modified and shipped, under the direction of Springfield Armory, eighty Phase III weapons to aircraft manufacturers, Air Force, and Ordnance installations. Preliminary tests indicated complete success on this program; however, owing to the increased reliability of the weapon, another major problem, which has always existed, became the prime point of work during this period. This problem is that of 'cook-off.'
Cornell Aeronautical Laboratory and Armour Research Foundation conducted extensive tests to determine cook-off information pertinent to the number of rounds which can be fired prior to cook-off, plus the determination of the cook-off phenomenon in the 20mm cartridge. M505 fuse tests demonstrated that detonation of the fuze booster, and consequently the entire projectile explosive train, can be initiated by fuze detonator cook-off. This fuze defect is extremely significant since the detonator is the most sensitive round element (it cooks after the fewest number of rounds are fired).
Methods of reducing the cook-off hazard were being reviewed by Cornell. In conjunction with this, Armor Research Foundation modified the pneumatic feeder shaft retracting device for the M39 Gun. The Feeder Shaft retracting device is a mechanical device which is operated automatically when gun temperature reaches the cook-off range. The sensing circuitry operates a pneumatic valve which retracts the feeder shaft then charges out three rounds making the weapon cook-off safe. When safe temperatures are reached, the device automaticaIn addition to the development of a thermal barrier system to prevent cook-off in the first and second ram positions of the drum, Ford has initiated a program whose general goals are to increase the thermal capacity of the drum body and the rate of heat transfer within it. This will allow an increase the thermal capacity of the drum body and the rate of heat transfer within it. This will allow an increase in the total number of rounds which can be fired before the cookoff point is reached. Calculations show that the heat capacities of the drum can be substantially increased by storing water in the drum body itself. During normal operation, the H20 will become violently agitated, which in turn will cause rapid heat distribution. High altitude flights would probably result in formation of ice in the drum reservoir. To prevent mechanical damage to the drum, a suitable quantity of antifreeze will be added to the H20 within it. A planned variation of the experimental drum design will be use a pressure relief valve instead of solid plugs. With such an arrangement, all of the H20 and antifreeze mixture can be evaporated, thus absorbing large quantities of heat from the drum. It is expected that steam ejected at this location will form a thermal barrier at the point of greatest heat transfer into the drum during normal weapon operation."
1JAN56 - 30JUN56 - "20MM Polygon Rifling - The first M39 20MM barrel with polygon type rifling (9 sides) fabricated by Pontiac was test fired for erosion life. The bore surface of subject barrel did not change in dimension or appearance and the plating appeared satisfactory. Due to the first attempt to fabricate and plate this type of rifling, inadvertently, the bore diameter finished oversize and therefore no velocity tests were performed. The barrel was fired three (100 rd bursts) and one (75 rd bursts) with 15 seconds between bursts for cooling. Serious keyholing became noticeable at 289 rounds, probably due to erosion within the forcing cone, and the test was concluded. Further attempts were scheduled to determine the erosion and accuracy characteristics of polygon rifling....
A replacement combination barrel seal and forcing cone was designed and fabricated for the 20mm automatic gun M39A1 in an attempt to solve the problem of erosion in the forcing cone area and in order to locate the area of origin for keyholing. This seal also reduced the impact force at the breech end of the barrel and when eroded may be replaced instead of the completed barrel....
Armour Research Foundation attempted to combat cook-off problems by reducing heat impact to the drum. The most promising method was by use of a smear capsule. By the use of a smear capsule (silicon oil), the temperature measurements on the barrel and drum were reduced as much as 60% in burst firing.
Armour had under test a pneumatically operated feeder shaft disengaging device and electrical control sensing system. This will be a gun safety anti-cook-off device.
Ford developed an open cycle refrigeration system which shows promise in reducing cook-off. Refrigerant is admitted through drilled holes in the drum shaft, then passes through headers which are drilled paralleled to the drum shaft. Refrigerant evaporates and escapes through holes form the header to the periphery of the drum.
Springfield Armory designed and tested several anti M505 fuze damage devices. These designs consisted of cantilever spring, link feeder sprocket, rammer modification and feeder relocation. Preliminary testing indicated satisfactory fuze damage elimination by use of a cantilever spring and feeder relocation combination.
Pontiac Motor investigated the cold working process for the gas pist1JUL56 - 31DEC56 - Armour Research Foundation equipped four weapons with automatic temperature sensing and clearing controls for testing cook-off prevention devices. They also continued investigation to eliminate fuze nose damage by use of cartridge guide pawls of varying configurations at various feeded locations. Ford Motor Co. worked on this problem and continued modifications to the feeder assembly to prevent the ammunition from being deflected downward by primary ramming. They also shipped to the Armory one weapon which incorporated anti-cycling safety and thermal barrier devices. This weapon was test fired 400 rounds prior to shipment to the Cornell Aeronautical Laboratory for further cook-off study. Ford Motor Co. fabricated one prototype Phase IV weapon which was undergoing tests."
1JAN57 - 30JUN57 - "Armour Research Foundation conducted tests to determine feasibility of plugging gas cylinder vent. T-D curves taken on 125-round burst indicated that the slide was being buffered from 50th round upward causing rounds to be incompletely rammed into the chamber.
Initial test of Pontiac Motor Div. GMC, machined gas piston, fabricated from Ti-6A1.4V, indicated good endurance life of at least 10,000 rounds.
Ford Motor Co., continued test of Phase IV weapon and commenced fabrication of two additional models. Ford delivered to Springfield Armory two 30-round chargers for evaluation.
Development and testing of 'ring' seals at Cornell Aeronautical Laboratory and Spfld. Armory and of skirted seals at Ford Motor Co., was continuing.
Standard two-piece ball joint seals, with tapered I.D. on short section and plate with spring-loaded latch, for use mainly on McDonnell aircraft installation, was recommended for incorporation into production."
1JUL57 - 31DEC57 - "Development of an M39A2 slide assembly incorporating Ford Phase IV type rammer retention and positive contractor was continuing.
Springfield Armory quick-disconnect power cord for the harness was proven satisfactory. High units were forwarded to test agencies.
The following R&D recommendations were forwarded to Rock Island Arsenal:
a. Uniform heat treatment of extractor shaft.
b. Slide body with increased radii.
c. Switch cam with .06 increase to bottom of cam path.
d. Gas cap with eight flats replacing annular grooves.
Two each Ford Motor Company and Armour Research Foundation prototype 30-round charges were delivered to Springfield Armory. One Ford Motor Company charger was function fired and forwarded to Aberdeen Proving Ground."
1JAN58 - 30JUN58 - "Urgent delivery requirements, beginning in September 1958, for M39A2 Machine Gun parts necessitated working on these during the vacation period....
Studies were initiated to provide a suitable mount of fixture to the mount the M39A2 on a helicopter."
1JUL58 - 31DEC58 - "Studies to provide a suitable mount for the 20mm M39A2 gun on a helicopter were underway. Two F101 soft mounts were to be procured for adapting the M39A2 gun to a helicopter with delivery of mounts expected in early January 1959....
Development of the enclosed switch for electrical barness continued at Ford Motor Co. Redesigned units were fabricated and forwarded to all agencies for test. The development of various slide assemblies which featured cam insert and rammer retention and positive extraction continued.
Testing of preproduction rings seals manufactured by Pontiac Motor continued and acceptance of a ring seal into production awaited the results of an aerial test by the Air Force.
A spring type retainer for the drum shaft was designed, tested and recommended to Rock Island Arsenal for production and a spring type retainer for the barrel retention was designed and units were fabricated."
1JAN59 - 30JUN59 - "Preproduction ring seals were endurance fired for acceptance by the Air Force and found satisfactory. The ring seal and ring seal drum were incorporated into production. Tests to determine temperature characteristics of the ring seal in serial test in F101 aiVarious development items which have improved function or reliability were incorporated into ten guns for endurance tests. Components which have been recommended for production have been included to fire one weapon with the latest components for evaluation. Components incorporated into the uns were the quick-change barrel latch, drum shaft latch, gas piston with rings, Model #2 electrical harness (enclosed switch), spring-type firing pin, drum assembly with ring seal, Model #3 slide assembly with 12-point wrenching screws, modified with cam and modified feeder."
1JAN60 - 30JUN60 - "Transfer of Support Mission Technical Responsibility from Rock Island Arsenal to Springfield Armory was completed."
1JUL61 - 31DEC61 - "Support tests were conducted at Fort Rucker Aviation Board of 20mm M39A2 automatic guns on H34 helicopter."

Notes: "The basic principles of the calibre .60 Revolver Gun T130 (M38) and the 20mm version, the T160 (M39) Revolver Gun, were conceived in the Mauser Works in Germany after the end of hostility of World War II. The Germans fabricated several models, designated as the 20mm MG213C and 30MM MG213 and sent them to the United Kingdom. Two guns were shipped to the United States, one of which was assigned to Springfield Armory and the other to Wright Field in Ohio.
In 1946 Springfield Armory studied the German model and prepared a set of drawings from the measurements taken from the weapon components. Late in 1946 the Armory received a set of the German prints and Armour Research Foundation was engaged to undertake developmental work. In 1947 Armor rebuilt the MG213C, also studying British developments. (The Aden redesign of MG213) and continued to redesign and fabricate versions in 20mm for the next two years. In the latter part of 1949 Springfield Armory made complete layouts showing a calibre .60 model using the revolver-gun principle. In early 1950 Armour redesigned and fabricated two models in this calibre, and later in the year designed and fabricated a lightweight version, known as the T130 Model C.
Following further refinements by Armour, Springfield turned the drawings over to Ford Motor Company in 1951. This company fabricated the Armour Model D as T130 E1/T160E1. Later that year Ford redesigned the Armour gun in the T130E2/T160E2. With Springfield Armory coordinating the program, Ford redesigned the E1 weapon for limited production into the T130E3/T160E3, including a sheet metal receiver, and fabricated 380 guns in 1951 and 1952.
In late 1951, Springfield Armory turned the drawings over to Rock Island Arsenal for an educational program there. Late the following year, 1952, Ford, continuing improvement, redesigned the E3 gun, which was then designated the calibre .60 T130E4/20mm T160E4. The calibre .60 version along with the comparable Gatling calibre .60 was discontinued when it was found that this ammunition did not have room for adequate high explosive.
In 1952, Pontiac Motor Division, General Motors Corporation, was given a production contract for 30,000 M39 weapons. In 1955, the second buy for 1640 was given to Cline Electric Co. The industrial responsibility for this particular weapon was given to Rock Island Arsenal. However, during the production orders, there was a continuing product improvement contract at Pontiac under sponsorship of Springfield Armory. Ford and Armor also continued to improve the weapon to include the T160E5 and T160E6 models. Type classification was accomplished in 1953.
In 1954, a contract with Pontiac Motors was negotiated to retrofit all existing guns. In early 1955, the Department of Defense cut back production because of poor reliability of the weapon. Improvements continued up to and including 1959 when solutions to most problems were found.
In 1959, the industrial support mission responsibility for the M39 was transferred from Rock Island Arsenal to Springfield Armory. The Armory periodically fabricated repair parts to support the approximately 31,640 weapons that were manufactured and installed in aircraft (F86, F100, F10The development of the M39 Machine Gun encompassed many facilities which included Norman McDonald & Co. which designed, developed and fabricated the cartridge charger; Dixon Research Inc. which developed and produced links; Roy S. Sanford Co. which designed and also developed links; Colt Patent Firearms Co. which studied a number of the gun components; Winchester Repeating Arms Co. which evaluated the design; Cornell Aeronautical Laboratory which conducted cook-off investigations and design activities that eliminated cook-off hazard by the use of ring seals; Frankford Arsenal which developed and fabricated ammunition and Rock Island Arsenal which prepared the Ordnance Drawings and initiated the production program.
On December 3, 1963, at the request of Colonel C. H. Johnson, Director of Procurement and Production, Hqs, USAWC, Springfield Armory furnished a cost estimate on the fabrication of 600 M39A2 weapons at a maximum rate of 75 weapons per month plus spare parts. The basis for the estimate were: the use of existing tooling, gages and special machines and the current knowledge as to the general condition of same. Initial delivery of weapons would be twelve (12) months after the receipt of product drawings and a production order. As of December 1, 1964, no further action had been taken to activate an order for new M39A2 Machine Guns.
When Springfield Armory ceased its production of the M39A2 Machine Guns its activities were confined to the production of repair parts for the weapon.
Since 1964 continued product improvement resulted, by 1967, in the adoption of the M39A3 version, which differs in some components from the M39A2 gun. The current weapon was produced by the Hydramatic Division of General Motors. Total production of all models of the M39 gun was over 35,000 and while the gun was not mounted on new aircraft it was still in service (1975) on such aircraft as the F-5." - Chinn

"Ford did not respond to the first invitation to bid on production 20mm M39 guns, thinking it was to be a set aside for small business. This was a misunderstanding as the award was to the Pontiac Division of General Motors, certainly not small business. For a while it became necessary to send the Pontiac guns to Ford for application of a retrofit kit with the latest changes. One area of rapid design changes was the revolver chamber seals, which to be sufficiently flexible to allow for barrel flexing, otherwise gas escape rapidly washed away the breech face of the barrel." - Rayle.

"The Air Force 20mm M39 revolver gun was developed by the Army Ordnance Department from the German World War II Mauser MG-312 aircraft machine gun. The M39 is gas-operated, and it fires the powerful M50 cartridge with a 3,300 feet per second muzzle velocity The M39 uses its five hole revolving cylinder to allow time for the loading and unloading to keep up with its 1,200 round per minute rate of fire. The system functions reasonably well. The M39's cylinder is pushed against the rear of its barrel when the gun fires to stop gas leakage that could damage the gun or the aircraft it is mounted in. The M39 is an adequate aircraft weapon that is effective on either ground or air targets, but the Vulcan is a far superior weapon for most uses. Only enough M39's have been procured to equip the aircraft specifically designed to mount it. The gun is not suitable for ground use because it must have fast moving air to cool it. It will probably go out of service as the aircraft mounting it are retired from use." - Konrad F. Schreier, Jr.

References:
Chinn, George M. THE MACHINE GUN. Vol. V. Edwards Brothers Publishing Co. Ann Arbor, Mi. 1987.
Rayle, Roy E. RANDOM SHOTS: EPISODES IN THE LIFE OF A WEAPONS DEVELOPER. Merriam Press. Burlington, Vt. 1997.
Schreier, Jr., Konrad. F. GUIDE TO UNITED STATES MACHINE GUNS. Normount Technical Publications. Wickenburg, Az. 1975.

TO 11WI-12-3-14; TO 11WI-12-3-22.