Standardized, Never Entered Service: 3-inch Gun Motor Carriage M9

The US Tank Destroyer doctrine was well renowned for its use of turreted self-propelled guns, designated as “Gun Motor Carriage” by the U.S. nomenclature in World War II. The intended role for this anti-tank vehicle design was to quickly adapt to the rapidly mobile battlefield, as well as to intercept, defend, and secure strategic locations against the enemy armored forces. A turret could provide an effective and immediate response against threats from more than just a single direction… even if some vehicles did not totally fit into that description (see the M10 TD and M8 GMC and their manually-traversed turret).

But such a design choice did not deter the U.S. Army Ordnance’s attempt to go on a venture and experiment with building turretless gun motor carriages. Most of them usually ended up as no more than just a single-working prototype to demonstrate their performance in trials. However, few of them made their way into a working interim solution until a proper turreted vehicle could roll out into production. Such an anti-tank vehicle was to be armed with an ancient 3-inch (76.2 mm) anti-air gun on top of a modified M3 Medium Tank chassis. Enter the 3-inch Gun Motor Carriage M9.

The Birth of U.S. Tank Destroyer Command

It all started with the German panzer formation rushing their way to Europe as a part of the Blitzkrieg campaigns in 1939. Deploying armor combatants in such a quick succession had set a wake-up call for the United States Army to quickly action this new form of armored offensive maneuver. Opinions of the U.S. followed two major lines. The first was to counter the armored opposition by drastically reorganizing the army by replacing the traditional infantry-artillery team with tanks combined with close air support. The second approach retained the older arms in addition to an armored force, but delegated tank defense to specially organized anti-tank units.

The latter concept was chosen by the Lieutenant General Lesley J. McNair, General Headquarters Chief of Staff. On his recommendation, the Tank Destroyer Command was established at Fort Meade, Maryland on 1 December 1941. The main putting a strong emphasis on forming up a dedicated tank destroyer unit to destroy enemy armor, thus permitting U.S. tanks to concentrate on supporting allied infantry. This was done by the exploitation of breakthroughs to strike the enemy rear areas. In order to meet this goal, a highly mobile turreted motor carriage design was required, which would lead to the development of the 3-inch Gun Motor Carriage M10. Unfortunately, manufacturing difficulties delayed this vehicle from being completed in its originally intended time period.

Too Conspicuous of a Tank Destroyer – The T24 GMC

As such, Ordnance had proposed an interim turretless gun motor carriage as early as September 1941. This vehicle would mount the 3-inch M3 L/50 anti-aircraft gun on the chassis of an M3A2 Medium Tank, as outlined in OCM 17358 on 23rd October, designated as the 3-inch Gun Motor Carriage T24. The pilot vehicle was built by Baldwin Locomotive Works and delivered to Aberdeen Proving Grounds (APG) early in November 1941, a month prior to the Attack on Pearl Harbor. The anti-aircraft gun was installed in an open-topped hull using parts from the standard M2A2 anti-aircraft gun mount.

As delivered, there were numbers of interferences found by Aberdeen, so before being subjected to any tests, they added a number of stops to limit the gun traverse and elevation guidance. The official gun elevation limit was 15°, but the actual maximum elevation was set to 16½°, while the gun depression limit was -1°56′. 23 rounds were fired from the T24 in testing. The gun could fire an armor-piercing round at a muzzle velocity of 853 m/s (2800 ft/s). The conclusion was that the anti-aircraft gun mount was considered stable enough and strong enough to carry the gun on a mobile platform. Although the specification required the gun to be able to depress up to -5°, the gun could only go down up to barely -2°, and -3° with the aid of jacks. The biggest issue with the T24 was its extremely high gun mount placement, resulting in an extremely recognizable silhouette of a tank destroyer vehicle from a distance. Fortunately, all the issues of its limited gun guidance and silhouette could be rectified by the means of a complete redesign to the vehicle’s gun mount. Other suggested improvements included heavier armor, provision for greater traverse, full protection for the crew against small arms fire from the front and flank, and generally improved performance characteristics. As a result, in January 1942, the 3-inch M3 gun was sent back to depot and the T24 retrieved back by Baldwin for conversion into its next iteration as the T40 GMC. The T24 Gun Motor Carriage project was cancelled by OCM 18061 in April 1942.

Reaching Standardization – The M9 GMC

The 3-inch Gun Motor Carriage T40 development was hastily initiated on 31st December 1941 while the T24 was under trials by APG. The attack on Pearl Harbor left the U.S. Ordnance in a precarious situation to urgently develop a working model of the first United States’ tank destroyer vehicle. The Assistant Chief of Staff J-4 approved the manufacture of some fifty Gun Motor Carriages based on the M3A2 Medium Tank hull, mounting the 3-inch M1918 L/40 anti-aircraft gun, the predecessor of 3-inch M3, firing at a significantly lower muzzle velocity of 732 m/s (2400 ft/s). As the T24 had been cancelled, the M3A2 hull formerly used for the vehicle was adapted for the T40 instead. Baldwin modified the front hull to provide a lower gun mount and to increase the gun traverse. The tank turret and hull top remained omitted. The gun platform was mounted directly on top of the drive shaft, and the gun installed on a traversing base ring above it. The trunnion mount was cut down by about a foot (30.5 cm) and welded back together to lower the entire gun installation and decrease the silhouette. Due to the new gun mounting platform, the elevating mechanism had to be redesigned, including an elevation limiter by the addition of a stop on the elevating rack, and depression by the means of the recoil cylinder hitting the front superstructure plate. It also provided a substantially more horizontal traverse to the right more than the left from the center.

The pilot vehicle was sent to APG for testing on 11th March 1942, marked with a registration number of 4026934. The firing trial was done with a total of 10 rounds to adjust the recoil mechanism proof firing of the gun mount. The gun could traverse up to 10° to the left and 27° to the right, and elevate up to 29° and depress -8°. The short testing pulled several conclusions of the gun motor carriage. There were problems in regards to its crew operation, such as the lack of space between the breech block of the gun and the rear hull plate to allow full normal recoil of the gun as originally designed, prohibiting loading the gun at elevations above 10° at center traverse. Others like the inconvenient elevating mechanism of the gun  and the lack of sighting device for aiming the gun were also noted. Ammunition boxes were also thought to be placed in a rather awkward position. To solve these concerns, modifications for the M9 were authorized to permit loading the gun at all elevations up to 20°, relocate the elevating handwheel to allow one-man operation, and rearrange the ammunition racks. M6 Telescope was recommended as the aiming sight for the M1918, mounted on the T42 Telescope Mount.

The T40 was recommended for standardization as the 3-Inch Gun Motor Carriage M9, Substituted Standard, by OCM 18143 on 30th April 1942, with the original order of 50 vehicles reduced to 28 when it was learned that only 28 3-inch M1918 guns were available at the time. As recommended for standardization, the M9 was to have a crew of five (with provision for carrying two M1 submachine guns and three M1A1 carbines), and a comparable automotive performance to that of the M3 Medium Tank, albeit slightly improved by a decrease in weight. At the same time, the representative of the Tank Destroyer Command expressed concerns over its adoption, stating that the M9 did not meet the tactical mobility requirements  which were paramount for a gun motor carriage in tank destroyer battalions.

The T40 was formally adopted as the 3-Inch Gun Motor Carriage M9, Substituted Standard, in accordance to OCM 18225 in May 1942, and efforts were made to find a suitable facility to produce the additional 27 vehicles. Since so few were to be produced, it was difficult to order the contract, and in any case, it was believed that the manufacturing cost would be undesirable. There was no assurance that all 27 of the remaining M1918 guns were serviceable as well. Accessories, spare liners, and other spare parts for the guns were not on hand and there were no facilities currently available to accommodate. Furthermore, the vehicle could not be produced prior to the time the 3-inch Gun Motor Carriage M10 would arrive in a handful quantity, armed with the 3-inch M7 L/50 in a fully traversable turret.

Therefore, the Ordnance Department recommended the cancellation of the M9 Gun Motor Carriage in July 1942. Formal approval was recorded by the Ordnance Committee on 20th August 1942 in OCM 18689, and the vehicle was removed from the Book of Standards.

Specification

Manufacturer: Baldwin Locomotive Works
Combat weight: 24.1 tonnes (53,150 lb)
Crew: 5 (commander, driver, radio operator, gunner, loader)

Mobility:
Engine: Continental R-975-EC1 Whirlwind, 9-cylinder radial engine
Engine power: 400 hp at 2400 rpm
Power-to-weight: 16.59 hp/ton
Transmission: Synchromesh, 5 speed forward, 1 reverse
Suspension: VVSS
Top speed: 40 km/h
Reverse speed: -4 km/h

Armor:
Front
Upper: 50.8 mm (30°)
Middle: 38.1 mm (53°)
Lower: 50.8 mm (45°)
Sides
Upper: 25.4 mm (0°)
Lower: 25.4 mm (0°)
Rear
19.05 mm (10°)
Top
12.7 mm
Floor
Front: 25.4 mm
Rear: 12.7 mm

Armament:
Main gun: 76 mm M1918 L/40 (40 rounds)
Horizontal traverse guidance: 37° (10° left / 27° right, manual)
Vertical traverse guidance: +29° / -8° (manual)
Muzzle velocity: 732 m/s
Rate of fire: 8 RPM
Ammunition: M62 APCBC-HE, M79 AP, M42 HE

Sources:

• Aberdeen Proving Grounds – Tank Data: vol. 3.
• Hunnicutt, R.P. – Sherman: A History of the American Medium Tank
• Morran, Nicholas – Can Openers: the Development of American Anti-tank Gun Motor Carriages