U.S. GRENADE LAUNCHER M75 40mm U.S. GRENADE LAUNCHER M75 40mm Manufactured by Springfield Armory, Springfield, Ma. in 1965 The M75 is a single barrel power-driven, belt-fed, 40mm High Velocity Grenade Launcher. The M75 was designed as an area suppressive fire system for helicopters. The gun itself fires a 40mm grenade cartridge from a linked metallic belt. The major components of the M75 weapon are the drive motor, drum cam, receiver, barrel and feed tray. The weapon is powered by a 5/8 horsepower, 28 volt DC motor, and is so constructed that all phases of the weapon cycle are positively controlled by the drum cam assembly. The drum cam enclosed the planetary gear train, which reduces the high motor speed to the desired rate for the gun. The motor is mounted on the turret bracket and drives the drum cam through a flexible shaft. In this manner the motor is isolated from the weapon and it is not adversely affected by the recoil. Muzzle velocity 790 fps. Rate of fire 225 rpm. The M75 was only used in the M5 system. Tag attached to weapon states receiver broken. This specimen packed in shipping can. Markings: Plate: LAUNCHER, GRENADE M75 SERIAL NO. 0014 U.S. SPRINGFIELD ARMORY. In May 1967 weapon was appraised at $1,127.00. HISTORICAL SUMMARY OF ACTIVITIES: 1JUL64 - 30JUN65 - "Launcher, Grenade, 40mm, M75 - An order was received in August to make 64 M75 Launchers and spare parts, the first two for delivery in December. Quantity on order was increased twice to a total of 145 weapons. Parts were made by tool room methods, requiring highly skilled personnel. Top setup men were assigned to develop mechanical setups and measuring procedures as no gages, special fixtures or tools were provided. With a lead time of only 45 days and some 112 machining operations on the receiver, the initial delivery of two weapons in December was made. Effective use of numerically controlled machines, overtime, and the effort and cooperation of all involved in the program made this possible. At the end of June, work was on schedule with a total of 61 weapons delivered. All component design changes for improved weapon performance and life had been incorporated in the 25 weapons delivered in June. Future Launchers will have the new barrel with shallow lock slot to increase strength of breech areas. The Barrel was the most difficult component to make. In rifling, even Carbide broaches broke down and produced grooves as though the material was flaking apart. Analysis of the steel showed that material met specifications. In order to meet schedules, barrels were rifled with a single cutter, a very expensive method. Even this method gave trouble with tool life. Rea recommended purchase of ten barrel blanks made of leaded steel but no material of this type had been found. A new set of broaches for four passes was being made, to be completed in August 1965. In attempting to procure castings for the Cam Firing Control, only one company (Waukesha Foundry of Waukesha, Wisconsin) was willing to accept the order. When problems arose an Armory representative visited this foundry and found that the contractor did not have equipment to check the Cam Track which was giving trouble. A casting furnished by the contractor was partially machined, the cam track laid out at the Armory and the casting then returned to the foundry for use as a master to correct their pattern. The next lot of castings should be of better quality. Procurement of the Gear Follower was also a problem, with rejections up to 90% from one company. Later shipments showed great improvements." 1JUL65 - 30JUN66 - "The schedule for this weapon was met except for fifteen launchers where were held up by late delivery of firing control castings. These castings were a troublesome item to procure, and throughout this order the Armory was short of castings because of single-source procurement. When requests for bids sent to 27 foundries received no response, the Armory started fabrication of four Firing Control Cams of steel tubing for test. This optional manufacturing method showed promise, with a unit cost of $20 less than the casting method. Parts are scheduled to be ready for test in September. New tooling was developed to eliminate some of the more expensive machining operations on the Receiver. These included broaching the cartridge open and milling two internal barrel slots or tracks, replacing costly slotting operations. Barrel rifling problems persisted into the FY 1965. New carbide broaches did not work out as expected, and the present order will be completed using the conventional rifling method and a single point rifling cutter. Broaching this barrel with wafer-type cutters had proved satisfactory in tests, but because of the Armory's closing, equipment for this operation was not obtained. Barrels made from 4147 leaded bar stock and from Chrome-Molybdenum-Vanadium bar stock to check machinability and durability against barrels from rotary forgings were tested. The barrels of leaded steel spilt at the breech end. Those from Cr-Mo-V steel tested satisfactorily, and the barrel drawings has been changed to permit manufacture from this steel in lieu of rotary forging. By the end of December, the contractor had overcome his major problems with the Gear Follower which had been a constant source of trouble since the beginning of the order, and deliveries have been improved." 1JUL66 - 30JUN67 - "Engineering Support M75 - XM79 - The following devices were designed in-house and working drawings were provided to the U.S. Navy for use on the XM129 Grenade Launcher: 1. Special feed tray to permit firing the low velocity ammunition as well as the high velocity. 2. Mechanism to provide instant interchange between electric motor operation and manual hand-crank operation. 3. A dynamic braking unit to guarantee that the launcher barrel will always stop in the forward (safe) position when the firing trigger is released.... Manufactured of parts for this Launcher was finished by the end of December, except for the Firing Control Cam, eighty-five of which has to be made for weapon assemblies and for spare Cam and Cover Assemblies. The Armory's situation had prevented extensive tests of the experimental Firing Control Cams made of steel tubing, and a complete t was not developed. Cams to complete the order, therefore, had to be made from castings. The last weapons were delivered in February, despite some delay caused by the Armory's phase-out situation and conflicting priorities. Machining of spare cams was finished by March so this item need not be moved to the Water Shops. No particular problems arose; however, was overtime into early CY 1967 was needed to complete the work.... Manufacture of parts for this launcher was finished by the end of December, except for the Firing Control Cam, eighty-five of which had to be made for weapon assemblies and for spare Cam and Cover assemblies. The Armory's situation had prevented extensive tests of the experimental Firing Control Cams made of steel tubing, and a complete process was not developed. Cams to complete the order, therefore, had to be made from castings. The last weapons were delivered in February, despite some delay caused by the Armory's phase-out situation and conflicting priorities. Machining of spare cams was finished by March so this item need not be moved to the Water Shops. No particular problems arose; however, use of overtime into early CY 1967 was need to complete the work." Notes: "The first patents that resulted in the M75 were applied for in 1961 and in that year the development and production of the M75 gun and its special link was begun by Philco-Ford. The work continued and resulted in the production of 500 guns. As part of the program for Army Aircraft Armament, the 40mm grenade launcher for UH-1 series helicopter to provide fire suppression, self-protection, and short duration air-to-ground fire support. Two rounds of ammunition were developed for the launcher, the M384 high-explosive cartridge and the M385 practice cartridge. Their designs were based, respectively, on the high-explosive and practice 40mm grenades for the M70 shoulder fired launcher. The cartridges have velocities of about 850 feet per second, considerably higher than the original design. These new rounds were accurate to a range of 2,045 yards. The M533 point-detonating fuze was employed in the XM384 and XM385 projectiles. The fuze was developed to operate under much higher setback forces than could be withstood by the original fuze used in the M79 40mm shoulder fired grenade launcher. This results from the fact that the XM75 grenade launcher had considerably greater chamber pressure than the M79. Because the M75 weapon was fired from a helicopter, the projectiles with the later fuze was designed to hit nose first. Though constructed to function primarily by nose impact, the fuze also functions on graze impact. For safety in firing, it had a delayed-arming distance of 60-120 feet from the muzzle of the launcer. Both types were detonator safe, and would not become armed until they were acted on by both setback and centrifugal forces. Work on the M75 weapon ceased in 1967 when the XM129 was sufficiently developed to take its place. The major problem with the M75 was the torque created due to the barrel being below the operating drum. In the XM129 the barrel is concentric with the drum and the excessive torque is thus eliminated.... Test Results - Ground and air tests of the prototype installation, which culminated in a successful aerial firing demonstration with high explosive rounds in June 1961, was followed by extensive redesign and tests of the entire armament subsytem. The armament supplies 30 seconds of continuous fire with controls for traverse and elevation. Since the accuracy of point fire was not required and since the operator can direct fire according to his observation of strikes, the sight mechanism met the Army's sighting and aiming requirements. Despite success of these texts with a single 40mm launcher in the nose of a helicopter, the Combat Development Command indicated a preference for dual installation in the UH-1B aircraft. The development of this dual began in fiscal year 1964, while continuing development of the M5 system. However, the requirement for a dual installation for the M75 was dropped because of the development of the more advanced XM129 40mm grenade launcher. The final report on the Initial Production Test of the M5 system was published in late 1967 and recommended the redesign of some parts to improve the life of the system." - References: Chinn, George M., THE MACHINE GUN. Vol. V. ,Edwards Brothers Publishing Co. Ann Arbor, Mi. 1987. Other Picture: Springfield Armory M75 40mm Grenade Launcher on XM9 armament subsystem Photos and information from: Roger Desbois

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