Springfield Armory Museum - Collection Record
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|Title:||GUN, MACHINE - U.S. MACHINE GUN T17E3 AIRCRAFT .60 SN# 5|
|Maker/Manufacturer:||FRIGIDAIRE DIV. OF GM|
|Date of Manufacture:||1945|
|Catalog Number:||SPAR 2828|
|Measurements:||OL:130.8CM 51 1/2" BL:|
U.S. MACHINE GUN T17E3 AIRCRAFT .60 SN# 5
Manufactured by Frigidaire Division of GM, Dayton, Oh. - The cal. .60 T17E3 machine gun is a recoil-operated, air-cooled, belt-fed weapon. The belt is of the metallic link disintegrating type. The gun can ben prepared for either left-or right-hand feed. Firing is by means of a solenoid, but a manual release for the sear is also provided. The weapon is provided with a shock-absorbing adapter which also serves as a front mounting. The gun can be charged by an electric charger mounted in the cover, by an auxiliary charger fastened to either side of the receiver, or by a manual charger assembled into the cover. Cyclic rate of fire: 600 rpm. Barrel is missing on this specimen.
Plate: PART NO. T17E3/TYPE 5/FRIGIDAIRE DIVISION/GENERAL MOTORS CORPORATION. 5 on numerous small parts.
Notes: CALIBER .60 T17E3 - Beginning in September 1943, attention was concentrated upon the manufacture of seven T17E3 guns with both mechanical and electrical ignition. The electrical ignition mechanism was modeled upon that of the German 20-mm gun. A program was also initiated for the development of a bolt with the sear forward. It was expected that this, together with electrical ignition, would permit synchronization of the gun when either electrical or percussion primed ammunition was used.
On 15 December 1943 because of increased interest by the United States Navy, this order was increased to ten guns.
On 16 November 1943, Frigidaire was requested to develop four auxiliary gun charges to be used as a standby device in case of the failure of the electric charger.
Because of a requirement placed with the Ordnance Department by the Navy Bureau of Ordnance for delivery of 2,500 caliber .60 aircraft machine guns with auxiliaries and spare parts in 1944, limited procurement of the T17E3 was initiated 2 December 1943 and approved 16 December 1943. This procurement was immediately begun even though preliminary service tests of the T17E3 had not been held. It was recognized, when the decision was made to rush the preparation for manufacture of the caliber .60 machine gun at Rock Island Arsenal, that the expenditure of large sums of money would be required to put changes in the design of components into production. However, it was felt that a major portion of the tooling for the gun would ultimately be usable, and that the plan followed would save priceless time in getting fully developed guns into the hands of the Army Air Force and the Navy Department. Rock Island Arsenal was commissioned to make these guns. The only unit to fail consistently, so far, was the bolt head. Redesign of this unit was immediately undertaken. The breech ring, firing pin, bolt head buffer springs and a few minor components had broken but it was believed that improved metallurgy for these components would satisfactorily increase their life.
This gun weighed 135 pounds, including recoil mounting adaptor, electrical charger and solenoid, and feed mechanism. The rate of fire was 600 rounds per minute, and the muzzle velocity was 3,500 ft/sec. The gun was 92 inches long and had a maximum trunnion reaction of 3,500 pounds.
At the request of the Army Air Forces the original procurement was increased to 5,025 guns by Ordnance Committee action on 23 December 1943. Twenty-five guns of this procurement were allotted to the Ordnance Department for experimental purposes.
Two T17E3 machine guns manufactured by Frigidaire were delivered to Aberdeen Proving Ground 29 December 1943 for test. The breech ring on Gun No. 2 broken after a few hundred rounds, severly damaging the gun. Gun No. 1 successfully fired several thousand rounds. The gun fired erratically with frequent and long hesitations. These hesitations were later found to be caused by stubbing of the bullet point against the breech face of the barrel. A 25-round burst was considered good. During January 1944 a research project was established with the Battelle Memorial Institute, Columbus, Ohio, for the purpose of making recommendations on heat treatment and otheDuring February and March 1944, Frigidaire engineers decreased the weight of the bolt to five pounds, increased the diameter of the bolt head roller trunnions to .656 inches, removed the link stripper from feedway since it was found unnecessary, and added a cartridge hold down lever and a feed pawl control lever to the feedbox. The feed pawl control lever prevents a round from being fed into the feedway by the feed pawls while another round is already over the feed slot in front of the holding pawls. This is accomplished by a linkage attached to the feed pawl control lever which lifts the feed pawls and prevents them from engaging the feed belt when a round is already over the feed slot and forward of the holding pawls. The cartridge hold down lever positions the round to the feed slot and holds the round down as the bolt head drives it forward.
In April and May 1944 this gun was tested and it fired 100 rounds at a uniform cyclic rate without a stoppage. The cyclic rate was approximately 600 rpm. Malfunctions were reduced to one percent and breakages to 0.3 percent. On 13 March, regular weekly conferences were initiated by the Small Arms Development Division, Research and Development Service, to integrate all information on the progress of the project. This conference was attended by representatives from Field Service, Industrial Service, Office of Air Ordnance Officer, Navy Bureau of Ordnance, Frankford Arsenal, and occasionally by representatives from Aberdeen Proving Ground, Rock Island Arsenal, and Frigidaire. In these meetings all changes in the gun design were discussed and transmitted to the Industrial Service, which in turn sent the information to Rock Island Arsenal. The minutes of these meetings, which were held weekly until 15 January 1945, bi-weekly until 27 August 1945, and monthly thereafter, are filed in the Small Arms Development Division, Research and Development Service, and in other Ordnance establishments.
A general description of the T17E3 gun is as follows: It consists of a receiver supporting a cover, breech ring and barrel. With the receiver slides a bolt and bolt head. In the cover is a cam tube with a helical slot cut in it. Riding within the cam tube, guided by the helical slot, is the drive bushing with incloses one end of a drive spring. On the outside of the came tube near its forward end is cut a spur gear which engages the rack that operates the feeding mechanism. When the cover is closed, a stud on the drive bushing engages a raised portion of the bolt and in operation the bolt, bolt head, and drive bushing travel together. The firing pin is fastened in the front of the bolt and projects into the bolt head. The charger retracts the drive bushing which in turn pulls the bolt back on the sear which is located at the rear of the receiver. When the sear is released, the bolt goes forward, the bolt head is rotated and locked by the cams on the breech ring and bolt. As the bolt head rotates, it retracts in the bolt. This causes the firing pin to protrude, ready to fire the primer. A buffer spring and a buffer plate are mounted in the buffer block at the rear of the receiver. This assembly arrests the reward movement of the bolt assembly and with the assistance of the drive spring returns this bolt assembly to battery during automatic fire.
THE ELECTRIC RELOADER - The electric reloader developed by the General Electric Company for the T17E3 gun is an apparatus designed to replace the manual reloading mechanism in the T17E2 gun with equipment that will clear stoppages automatically and reload the gun when required. The standard 27.5 volts DC current for airplanes is used as the power source for the operation of this charger and its controls. This reloader includes all parts and assemblies necessary for its operation. The cable and cross pin in the cover (used in manual reloaders) are replaced by the reloader drive unit whiThe adaptor group houses a ball bearing thrust nut which rides on the lead screw. As the lead screw is rotated by the motor the adapter group is driven to the rear along the screw, picking up the drive bushing and retracting the bolt until it engages the sear. This operation also compresses the drive spring. The adapter group is then returned to the front of the lead screw ahead of the bolt by reversing the direction of rotation of the motor by means of a telltale and switch located in the receiver.
The electric operation of the sear is accomplished by the firing solenoid which is bolted to the back of the buffer block. THe armature in the solenoid is attached to a lever arm which actuates the link bar which in turn is fastened to the sear mechanism. When the solenoid is energized, the link bar is moved to the rear. This depresses the sear and frees the bolt, which is forced forward by the drive spring firing the gun.
The motor on the drive unit is series wound with a reversing field and is rated 0.4 HP at 10,000 rpm at 25 amperes and 24 volts DC.
BASIC BARREL DEVELOPMENT - Development work on the gun barrel has been carried on by Springfield Armory and Frankford Arsenal. The initial requirement for the T17 gun called for a quickly detachable and replaceable barrel. This feature was taken by Frigidaire from the German MG 151 design. Springfield Armory made the first barrels and integrated the barrel design. After the initial barrels were fired, Springfield Armory worked on improvements in metal strength characteristics and heat treatment of barrels. Frankford Arsenal developed the original data on bore diameter and depth of rifling and determined that 590 grains of power (IMR 4996) would yield an instrumental velocity at 78 feet of 3,500 +/- 30 ft./sec. with a 1,200 grain projectile. In March 1944 a groove diameter of .604 + .002 and a land diameter of .588 + .002 with one caliber of free travel were adopted reduced the peak chamber pressure approximately 2,000 pounds per square inch and the initial muzzle velocity by approximately 100 ft./sec. The standardized depth of rifling of .007 inch to .009 inch was an increase of .004 inch in depth over the first barrels used.
At approximately the same time, it was desired to increase the capacity of the cartridge case and to make it more nearly cylindrical to facilitate manufacture and stowage. Accordingly, modified chambers and cartridge cases were designed and tested. The old cartridge (T1) was characterized by a .050 body taper, the new (T22) by a .035 body taper. The changes in the chamber and rifling caused some unavoidable delay in barrel production, which was soon overcome.
BORE COOLING DEVICE - Research on a bore cooling device for the caliber .60 machine gun was actively pursued at Purdue and Frigidaire. The average barrel life for this gun was 400 rounds when fired in 25-round bursts with a 2 minute cooling interval between bursts. In February 1944, the fundamental studies were completed at Purdue and in March 1944, Research and Development projects were accepted by the Bendix Aviation Corporation and the Diesel Engineering and Manufacturing Corporation. It was found that a bore cooling device for this weapon was highly desirable to increase the barrel life.
Concurrently other methods of increasing barrel life were being investigated and before a satisfactory bore cooling device was obtained, the life of the barrel had been considerably increased by the use of chromium plating and stellite liners. Because of the success of the liner and plating program, the bore cooling project soon became of minor importance.
FIRST BOLT FOR ELECTRICALLY PRIMED AMMUNITION - During April 1944, two T17E3 machine guns were completed at Frigidaire and fitted with bolt assemblies to fire electrically primed ammunition. These bolt assemblies were adapted from the bolt used in the German MG 151 to fire electrically primed ammunition. The electrical connector between theThis experimental work was continued at the Caliber .60 Machine Gun, T39.
TM9-231- CAL. .60 MACHINE GUN T17E3. War Department. June, 1944.
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