Category: WW2 US SPG Prototypes

United States of America (1942-1943)
Tank Destroyer – 1 Prototype of Each Built

Based on the chassis of the newly available M3A3 ‘Stuart’ Light Tank, the 3 in Gun Motor Carriage T56 and 3 in Gun Motor Carriage T57 were prototype American tank destroyers developed at the end of 1942. Both vehicles were to-the-doctrine American tank destroyers, featuring light armor, a powerful 3 in gun, and acceptable mobility. The two tank destroyers were identical in every respect except for their choice of engine. Testing revealed, however, that the vehicles had lengthy design flaws and both projects were subsequently canceled in February 1943.

The Origin of the American Tank Destroyer

While the concept of a mobile, lightly-armored, and heavily-armed tank destroyer was first explored during the 1930s, the United States Army recognized the need for it in mid-1940, when Germany successfully invaded France. Although at a numerical disadvantage, the Germans were able to break through French defenses by striking with concentrated massed armor attacks. French anti-tank guns, unable to halt the rolling waves of steel, were overwhelmed at key positions. The United States, having witnessed the fall of France, was determined not to meet the same fate. To avoid falling victim to these aggressive, concentrated panzer strikes, which were known as blitzkrieg [Eng. lightning war], the United States pursued the concept of the tank destroyer force.

After years of argument, work on Field Manual 18-5 “Tank Destroyer Field Manual, Organization and Tactics of Tank Destroyer Units” began in January 1941. This manual was the first definitive account of both American tank destroyer doctrine and the key characteristics of tank destroyers. Although slightly paradoxical and dated by 1942 standards, FM 18-5 was an instrumental piece of American tank destroyer history, single-handedly shaping the design of dozens of vehicles, both iconic and obscure. The official tank destroyer doctrine was summarized in FM 18-5 as follows:

“Tank destroyer units are especially designed for offensive action against hostile armored forces. When supported units are engaged in offensive action, tank destroyers protect them against armored counterattack and thus allow full exploitation of their success. When a supported unit is engaged in defensive action… organic antitank weapons of front line units are used [as the] first line of defense; tank destroyer units form the mobile reserve” (7).

Tank destroyers can best be described as “offensively defensive” weapons. Although well capable of destroying enemy armor, tank destroyers were not expected to lead the charge against defending infantry or artillery. In both defensive and offensive operations, the tank destroyer force was to remain behind the front line and act as a mobile reserve ready to halt a counterattack or sudden breakthrough.

FM 18-5 then goes on to describe the notable characteristics of a tank destroyer:

“The characteristics of tank destroyer units are mobility and a high degree of armor-piercing fire power, combined with light armor protection; strong defensive capacity against attacks of combat aviation; and flexibility of action permitted by generous endowment with means of communication” (7).

Using the above list as a guide, General Andrew Bruce, the major proponent of self-propelled anti-tank guns within the War Department and head of the Tank Destroyer Tactical and Firing Center, submitted a set of criteria in February 1942 that defined his view of the “ideal tank destroyer.” This vehicle would be mobile, lightweight, easy and cheap to produce, simple to repair, have a crew of five, and be armed with a three-inch gun. As demanding as these specifications were, a vehicle fulfilling almost all of these criteria did eventually enter service. Designated as the 76 mm Gun Motor Carriage M18 ‘Hellcat,’ this vehicle was everything General Bruce was looking for. There was, however, one small problem: the Hellcat would not see service overseas until summer 1944.

In the meantime, expedient tank destroyers were pressed into service including: the 75 mm GMC M3, an M3 half-track mounting a First World War-vintage M1897A4 field gun; the 3 in GMC M10, a modified Medium Tank M4A2 ‘Sherman’ hull fitted with a fully-rotating turret mounting the powerful 3 in Gun M7; and, the 37 mm GMC M6, which fitted a 37 mm Gun M3 to the rear of a ¾ ton Dodge 4×4 truck. Designed to carry a gun into combat by any means necessary, these vehicles compromised speed, in the case of the M10, or firing arc and armor protection, in the case of the M3 and M6.

As famous as they are, these three vehicles just scratch the surface of American tank destroyer development during the Second World War. For each tank destroyer that entered service, ten were rejected. They came in all shapes and sizes, mounting 37 mm, 57 mm, 75 mm, 3 in, 76 mm, or even 90 mm guns on trucks, half-tracks, and an astounding array of tracked vehicle chassis. One project involved mating a powerful 3 in gun with the chassis of the newly-developed Light Tank M3A3 ‘Stuart.’

The M3A3 ‘Stuart’

First conceptualized in April 1942, the M3A3 ‘Stuart’ was the final mass-produced member of the M3 family. Compared to the older M3A1, the M3A3 featured redesigned frontal armor modeled after that of the M5 ‘Stuart.’ The vehicle also featured additional sponson fuel tanks, redesigned crew hatches, an upgraded radio, and an improved gun mount, amid a slew of other improvements. The increased weight of the M3A3’s hull armor meant that its final drive ratio had to be adjusted, reducing the tank’s acceleration. The M3A3 entered service in September 1942 and a total of 3,427 were produced.

Development

Always eager to take advantage of the newest tank developments, the Ordinance Committee immediately conceptualized two new 3-inch gun motor carriages based on the M3A3 chassis. Both vehicles were to remain as similar as possible. Their only significant difference was to be found in the choice of powerplant. The first tank destroyer, designated 3 in Gun Motor Carriage T56, was to be powered by a Continental W-670 Series 12 engine. This engine, which was quite similar to a standard M3A3’s, would significantly reduce the amount of time required to produce a prototype, allowing tests of the vehicle to commence as soon as possible. The second, the 3 in Gun Motor Carriage T57, featured the much ‘beefier’ radial Wright R-975-C1.

Mounting a large gun on such a small chassis necessitated a drastic redesign of the M3A3’s hull. A turretless design was the only option, so the 3 in Gun M7 was placed on a pedestal mount at the very rear of the chassis, presumably to minimize the frontal overhang of the gun. Accordingly, the M3A3’s engine compartment was moved to the middle of the hull directly behind the driver’s compartment.

To provide space for the gun crew, a folding metal plate was installed just in front of the ammunition box at the rear of both vehicles. When in firing position, this plate could be folded down and used as a loading platform. When the vehicle was on the move, this plate would be folded up. Where the rest of the crew would sit in the meantime remains a mystery, although it can be assumed that they would have followed behind the tank destroyer in an accompanying light vehicle.

Gun Motor Carriage T56 Testing

The first T56 GMC was completed by the American Car & Foundry Company on November 10, 1942 and delivered to Aberdeen Proving Ground. Before testing, a few modifications were made to the vehicle. A gun sight was mounted to the left of the gun and an expedient gun shield was mounted on November 17 to provide some protection for the gun crew. Despite plans to mount it on both vehicles, the T56 would be the only vehicle to receive the gun shield. Testing began in earnest, but, in the words of author Nicholas Moran, “so many flaws were discovered that the writers of the report ran out of words to enumerate them” (117).

Aberdeen Proving Ground concluded that:

• The vehicle’s poor weight distribution and cramped driving compartment made steering the vehicle unnecessarily difficult.
• The T56’s rear center of gravity placed significant strain on the idler wheel.
• The fuel tanks, which were located directly behind the driver and assistant driver, presented a severe fire hazard.
• Because the ammunition box was mounted directly in front of the engine compartment, the entire vehicle had to be disassembled for even the smallest of tune-ups.
• The engine’s cooling system was unacceptable. Parts of the engine reached up to 580° F after an automotive test.
• The gun shield offered no side or rear protection for the gun crew.
• The vehicle was not equipped to carry the gun crew when in non-combat situations.
• Ammunition stowage, which was 40 rounds, was considered inadequate.
• It was impossible for a single crewmember to simultaneously traverse and elevate the gun due to the awkward positioning of the handwheels.
• When firing the gun, the vehicle would rock violently in such a way that rapid, aimed fire was impossible.

The sole positive characteristic highlighted by Aberdeen Proving Ground was the vehicle’s mobility. The T56 was capable of reaching a top speed of 39 mph (63 km/h) in 54.1 seconds, which was rather impressive for the time. One upside, however, does not a functional tank destroyer make.

By December 1942, Aberdeen concluded in their official report that an almost complete redesign would be necessary to render the T56 fit for service. This prospect was, understandably, laughably impractical as the superior 75 mm Gun Motor Carriage T67, a prototype of the 76 mm Gun Motor Carriage M18 ‘Hellcat,’ was nearing completion anyways. In addition, there were worries about the engine becoming overburdened if the vehicle was redesigned.

Gun Motor Carriage T57 Testing

On December 8, 1942, shortly after the T56’s poor testing report was submitted, the T57 Gun Motor Carriage was finally delivered to Aberdeen Proving Ground. Before testing could commence, some slight repairs were made to the vehicle’s automotive systems, which had arrived with some damage. Once testing of the vehicle finally began on December 17, 1942, some marked advantages over the T56, if only in the automotive department, were discovered. The boost in mobility was due to the T57’s new engine, the Wright R-975-C1, which was capable of producing an impressive 400 gross hp. During tests from January 5 to January 11, the T57 was able to reach a top speed of about 50 mph (80 km/h) on paved roads and 30 mph (48 km/h) on softer dirt.

Testing did not go off without a hitch. Concerns were raised about failure of the break bands, which would be subject to increased wear as a result of the much stronger engine. These fears would be realized when, in the middle of testing, one brake band snapped and the other wore heavily. The cause of this failure was determined to be issues with the transmission and oil pump, which exposed the bands to excessive heat and limited lubrication. After replacing the faulty parts, testing continued without further issue.

Unfortunately, the T57 shared quite a few issues with the T56. The problems with the firing platform and weight distribution remained unsolved and engine access was actually made even more difficult. The entire engine would still have to be removed for any amount of maintenance, no matter how small. By the end of its testing, the T57 had been driven a grand total of 887 mi (1,427 km) between 17 December 1942 and 13 April 1943.

Cancellation

On December 3, 1942, the Special Armored Vehicle Board, a board of six officers tasked with evaluating experimental armored vehicles, submitted their own report on the T56 Gun Motor Carriage. Their thoughts were quite similar to those of Aberdeen’s:

“In the opinion of this Board a gun motor carriage for tank destroyer use must be a self-contained unit capable of carrying the gun crew and all necessary ammunition and equipment without recourse to an accompanying vehicle. It is not believed that so large a gun as a 3” or 76mm can so be mounted on a light tank chassis as to provide either a stable firing platform or suitable characteristics just referred to.

None of the using services represented on this Board (The Armored Force, Tank Destroyer Command, and The Cavalry) desires Gun Motor Carriage T-56.

This Board finds no reason for further consideration of Gun Motor Carriage T-56 ‘for service use or for further use or for further service test or to outline further developments to be pursued’ by the United States Army.

This Board unanimously recommends the termination of further development of the Gun Motor Carriage T-56 for the US Army.”

Special Armored Vehicle Board, 3 December

In early February 1943, in recognition of the negative reports by both the Special Armored Vehicle Board and Aberdeen Proving Ground, the Ordinance Department recommended that the development of 3 in Gun Motor Carriages T56 and T57 be canceled. On 24 February 1943 this decision was finalized and the tank destroyers were to be kept at Aberdeen. The fate of the vehicles past this point is unknown, although testing of the T57 did continue until 13 April despite its cancellation. The T56 and T57 were presumably scrapped by Aberdeen Proving Ground in the late 1940s alongside other prototype armored fighting vehicles from the Second World War.

Design

Firepower

Gun Motor Carriages T56 and T57 were fitted with the same gun, the 3 in Gun M7. This gun, originally developed for anti-aircraft use, was adopted for use as an anti-tank gun on quite a few vehicles, including the 3 in Gun Motor Carriage M10 and Heavy Tank M6. It could shoot a variety of rounds, including M62 Armor-Piercing Capped Ballistic Capped (APCBC), M93 High-Velocity Armor-Piercing (HVAP), M79 Armor-Piercing (AP), and M42A1 High-Explosive (HE). Firing M62 APCBC rounds, the M7 could penetrate a maximum of 3.7 in (93 mm) of rolled homogeneous armor at a range of 500 yds (457 m) and an angle of 30º, which would have given the T56/T57 a fighting chance against nearly any Axis armor of the time.

When mounted on the T56 and T57, the gun could traverse 15º either direction horizontally. Vertically, the gun could depress up to -5º and elevate to a maximum of 25º. Due to the unusual positioning of the hand cranks, two crew members were required to simultaneously elevate and traverse the gun. Up to 40 rounds of ammunition could be carried in a box located at the rear of the vehicles.

There were also plans to fit an anti-aircraft .30 caliber M1919A4 machine gun to both vehicles, but this was never done. A planned storage of 500 .30 caliber rounds was specified. In addition, two M1 Carbines and 100 rounds of ammunition were to be carried within the vehicles for personal crew protection.

Protection

The T56 and T57 had rather unremarkable armor protection. For the most part, it would have protected the vehicles against heavy machine guns and smaller anti-tank projectiles from a distance. Direct fire from larger cannons, however, would have made short work of the vehicles. Their hull armor was equal to that of a standard M3A3 ‘Stuart,’ the front of which was 1 in (25 mm) thick and angled 48º from vertical. The gun shield fitted to the T56 was 1.5 in (38 mm) thick at the front and angled roughly 30º from vertical. It provided no protection for the rear of the vehicle, much to the dismay of Aberdeen Proving Ground. The T57, which did not receive the shield, had no protection for the gun crew at all.

Mobility

The T56 Gun Motor Carriage was fitted with a Continental W-670 Series 12 engine, which was capable of producing 288 gross hp @ 2,600 rpm. This 7-cylinder radial engine was a more powerful variant of the Continental W-670-9A fitted to standard M3A3s. Given that it weighed 16 tons (14.5 tonnes), the T56 had a respectable hp/ton ratio of 18 (19.9 hp/tonne) and a top speed of 40 mph (63 km/h). However, the vehicle’s paltry 50 gal (189 l) fuel capacity gave it a cruising range of just 70 mi (113 km).

The T57, however, was fitted with a much larger engine: the Wright R-975-C1. Although it increased the weight of the vehicle to 16.5 tons (15 tonnes), the Wright was able to output an astounding 400 gross hp @ 2,400 rpm. This gave the T57 an extremely high hp/ton ratio of 24.2 (26.7 hp/tonne) and a blisteringly fast top speed of 50 mph (80 km/h). Unfortunately, the fuel capacity and cruising range remained a disappointing 50 gal (189 l) and 70 mi (113 km) respectively.

Crew and Ergonomics

The T56 and T57 were each manned by a crew of 5 (driver, assistant driver, loader, gunner, and commander). The driver sat in the front left of the vehicle and the assistant driver sat in the front right. At the rear of the vehicle the gunner would be positioned to the left of the gun, the loader would stand directly behind it, and the commander would be stood to the right of the gun.

One of the T56/T57’s greatest issues was its poor crew comfort. The hull crew, the driver and assistant driver, were left without much room to maneuver and were seated directly in front of a massive fire hazard. The gun crew had no provisions at all, with the intent to have them follow in a separate vehicle. Both Aberdeen Proving Ground and the Special Armored Vehicle Board possessed an immense dislike for this particular fact, preferring instead that the entire crew be able to ride inside a single vehicle to ease the burden on logistics. The ergonomics of the T56 and T57 were, in short, horrible. Depending on the crew member, they were either poor or nonexistent.

Conclusion – A Comedy of Errors

The 3 in Gun Motor Carriages T56 and T57 were, if anything, prime examples of how not to build a tank destroyer. Although they theoretically filled the role of ‘tank destroyer’ quite well, both vehicles featured a bafflingly large list of deficiencies in almost every area, including firepower, reliability, crew comfort, protection, and mobility. Any effort spent attempting to improve either vehicle would have been in vain as superior gun motor carriages, most notably the 75 mm Gun Motor Carriage T67, were already well into development. The T67 would eventually become the highly successful 76 mm Gun Motor Carriage M18 ‘Hellcat.’ The T56 and T57, in contrast, would be unceremoniously canceled, scrapped, and forgotten, relegated to mere footnotes in AFV history. Save for a brief inclusion in Wargaming’s World of Tanks, both vehicles remain relatively obscure to this day, their histories mostly forgotten and influence largely overlooked.

3 in Gun Motor Carriage T56 Specifications
Dimensions (L x W x H)	16’10.75” x 8’3” x 7’9.5” 5.15 x 2.51 x 2.37 m
Weight	16 tons (14.5 tonnes)
Armament	3 in Gun M7 (40 rounds) .30 caliber M1919A4 machine gun – not fitted (500 rounds) 2 x .30 caliber M1 Carbines (100 rounds)
Armor	Gun Shield Front: 38.1 mm Side: 38.1 mm Rear: none Top: 12.7 mm Hull Upper front: 38.1 mm Lower front: 44.5 mm Side: 25.4 mm Rear: 25.4 mm Top: 12.7 mm Floor: 12.7 mm to 9.53 mm
Crew	5 (driver, assistant driver, loader, gunner, commander)
Propulsion	Continental W-670 Series 12, 288 hp gross @ 2,600 rpm, 18 hp/ton (19.9 hp/tonne)
Transmission	Synchromesh
Fuel Capacity	50 gal (189 l) 80 Octane
Cruising Range	70 mi (113 km)
Top Speed	40 mph (64 km/h)
Suspension	Vertical volute spring
Radio	SCR 510
Total Production	1 prototype

3 in Gun Motor Carriage T57 Specifications
Dimensions (L x W x H)	16’10.75” x 8’3” x 7’9.5” 5.15 x 2.51 x 2.37 m
Weight	16.5 tons (15 tonnes)
Armament	3 in Gun M7 (40 rounds) .30 caliber M1919A4 machine gun – not fitted (500 rounds) 2 x .30 caliber M1 Carbines (100 rounds)
Armor	Gun Shield – not fitted Front: 38.1 mm Side: 38.1 mm Rear: none Top: 12.7 mm Hull Upper front: 38.1 mm Lower front: 44.5 mm Side: 25.4 mm Rear: 25.4 mm Top: 12.7 mm Floor: 12.7 mm to 9.53 mm
Crew	5 (driver, assistant driver, loader, gunner, commander)
Propulsion	Wright R-975-C1, 400 hp gross @ 2,400 rpm, 24.2 hp/ton (26.7 hp/tonne)
Transmission	Synchromesh
Fuel Capacity	50 gal (189 l) 80 Octane
Cruising Range	70 mi (113 km)
Top Speed	50 mph (80 km/h)
Suspension	Vertical volute spring
Radio	SCR 510
Total Production	1 prototype

Sources

TM 9-726C (French) Char Leger M3A3
Stuart – A History of the American Light Tank by R. P. Hunnicutt
Seek, Strike, and Destroy US Army Tank Destroyer Doctrine in World War II by Dr. Christopher R. Gabel
Can Openers – The Development of American Anti-Tank Gun Motor Carriages by Nicholas Moran
M10 and M36 Tank Destroyers 1943-53 by Stephen J. Zaloga
M3 and M5 Stuart Light Tank 1940-45 by Stephen J. Zaloga
Альтернативный легкий самоход на альтернативном шасси | Юрий Пашолок | Дзен
WarWheels.Net – The Special Armored Vehicle Board (1942): Design, Development, Engineering and Production of Armored Cars (1940-1944)
3 Inch Gun M7 Information Page. | The Sherman Tank Site
Tank Destroyer Dead Ends
Light Tank M3 Stuart

United States of America (1942-1943)
Tank Destroyer – 1 Converted

Created in January 1943 as a proof of concept, the 75 mm Gun M3 on 75 mm Howitzer Motor Carriage M8 Chassis was an experimental American light tank later designated as a tank destroyer. As the vehicle’s name would suggest, this project involved mounting the 75 mm Gun M3 in the turret of the 75 mm Howitzer Motor Carriage (HMC) M8. While initial testing at Aberdeen Proving Ground was promising, the tank destroyer’s numerous flaws were discovered during further testing by the Armored Force Board and the Tank Destroyer Board. As a result, the entire project was canceled in June 1943.

Small Gun Problems

The most numerous American light tank available at the beginning of World War 2 was the M3 Light Tank, also known as the ‘Stuart.’ While the M3 was relatively reliable and agile, it lacked armor protection and firepower. The tank’s lack of armor was not unexpected, of course. To remain mobile, light tanks have to be, as the name would imply, light. In part, this is achieved by keeping armor to a minimum. The tank’s lack of firepower, however, was a much larger issue. The 37 mm Gun M5 of the ‘Stuart’ could handle lightly-armored targets, such as the Panzer II and early Panzer III, without many issues. However, combat in North Africa in late 1942 illustrated the obsolescence of the 37 mm gun rather clearly. Up-gunned and more armored tanks, especially Panzer IVs, were appearing more frequently and were proving to be much more resistant to 37 mm guns than earlier German tanks. In a frontal engagement, the M3 ‘Stuart’ stood almost no chance against the improved armor of contemporary German Panzers. In contrast, the larger 75 mm M3 gun used by the M3 and M4 Medium Tanks was still considered an effective anti-tank weapon despite the appearance of heavily armored German tanks. Therefore, in an attempt to increase the firepower of their previously poorly-armed light tanks, the United States began to pursue light tank designs mounting a 75 mm gun in a fully traversable turret.

The M7 Medium/Light Tank

The first attempt to create a light tank mounting a 75 mm gun began in July 1942 as a further development of the Light Tank T7. The T7 project was conceived in January 1941 as an improved replacement for the M2A4 light tank then in service. The tank was to feature frontal armor 1.5 inches (38.1 mm) thick, a vertically-stabilized 37 mm gun M6, a weight of 14 tons (12.7 tonnes), and a powerful engine, amid a slew of other improvements. However, by July 1942, the Armored Force requested that the 37 mm gun be replaced by a 75 mm gun to increase the firepower of the T7. The British 6-pounder (57 mm) anti-tank gun was also considered as a potential main armament but was dropped in favor of the 75 mm. This variant of the T7 was designated as the T7E5 and it featured the aforementioned 75 mm gun, a 450 hp engine, and cast armor up to 2.5 in (63.5 mm) thick.

However, as a result of all the improvements made to the T7E5 design, this ‘light’ tank now topped the scales at 27 tons (24.5 tonnes), almost twice its predicted weight of 14 tons (12.7 tonnes). Now falling outside the weight requirements of a light tank, the T7E5 was standardized in August 1942 as Medium Tank M7. However, tests conducted in late 1942 revealed that the M7 was inferior to the M4 Sherman. Both vehicles were effectively equivalent in firepower and mobility, though the M4 offered more effective armor protection than the M7. The T7 project was promptly terminated and eventually declared obsolete in January 1944.

The T21 Light Tank

Despite the ongoing failure of the M7, the Ordinance Department and Armored Board were unperturbed. They wanted an up-gunned light tank design even if the M7 was a failure. In August 1942, they held a conference at Fort Knox to discuss the specifications of their ‘ideal light tank.’ As a result of the conference, a design based on the Medium Tank T20 was proposed. This light tank design would feature the 75 mm gun M3 (or a more powerful gun if the chassis could support it), armor capable of stopping .50 caliber armor-piercing rounds, and the suspension of the M7 medium tank.

By February 1943, the vehicle had been designated as Light Tank T21 and its specifications had been further refined. The predicted weight of the T21 was 23.5 tons (21.3 tonnes) and the vehicle was to be fitted with a 76 mm gun. A maximum top speed of 35 mph (56.3 km/h) and a cruising speed of 25 mph (40.2 km/h) were estimated. The vehicle’s impressive mobility was a result of keeping armor to a minimum, with the thickest plates on the tank being just 1.125 in (28.6 mm) thick. While the T21 promised to be an agile and well-armed light tank, Fort Knox had their worries. When final drawings of the tank were submitted in March 1943, the predicted weight of the T21 had increased by another 2 tons (1.8 tonnes). Afraid of making the same mistake twice, Fort Knox shortly canceled the T21 light tank project before months of wasted time and money could result in another ‘obese’ ‘light’ tank.

Development

Previous attempts to develop a successful light tank design featuring a 75 mm main gun were not very fruitful. The M7’s hefty weight made it unfit for its role and the T21 was in danger of following the same path. The experience in North Africa illustrated a desperate need for light tanks mounting larger guns, so an expedient light tank design was pursued. This expedient design was to be based on the chassis of the M3 or M5 light tank and mount a 75 mm gun in a turret. The usage of a reliable, standardized chassis would help keep the weight of the vehicle down.

To test if it was even possible to mount and fire the 75 mm gun M3 from the chassis of a Light Tank M3 or M5, the order was given in late 1942 to mount the 75 mm gun M3 on the chassis of the Howitzer Motor Carriage M8. The M8 HMC used the same chassis as the M5 and was chosen because its larger turret ring and open-top could accommodate the 75 mm gun much more comfortably than the smaller, fully-enclosed turret of a standard M5.

For the conversion, a few modifications were made to both turret and gun. For the turret, the anti-aircraft machine gun and the small roof plate it was mounted on were removed to make more room for the turret crew. A hole was cut into the front of the turret to make room for the larger gun. However, this new opening was covered by the mantlet, meaning it was not visible in photographs taken of the vehicle. To ensure compatibility between the 75 mm gun and the recoil mechanism of the M8, a special adaptor was created and attached to the recoil system. However, this device prevented the breech block from closing automatically, slowing the gun’s rate of fire.

With these modifications made and the 75 mm M3 mounted properly in its turret, the modified M8 HMC was delivered to Aberdeen Proving Ground on 30th January 1943 for trials. Before testing, the designation ‘M8A1’ was stenciled on the front corner of the vehicle’s hull. While the name ‘M8A1’ or ‘M8A1 Gun Motor Carriage’ has remained a common misnomer for the vehicle, it was never official.

Tank Destroyer Board Testing

The trials at Aberdeen were so successful that a representative requested that the vehicle be delivered to the Tank Destroyer Board to be tested for service as a tank destroyer. In February 1943, the vehicle was tested by the Tank Destroyer Board to further evaluate its performance and compared to two standardized tank destroyers, the 3 in Gun Motor Carriage M10 and the 75 mm Gun Motor Carriage M3. During these tests, the vehicle was called ‘M8 Gun Motor Carriage’ by the Board.

The test results were mixed, with some positive aspects of the vehicle noted:

• Cross-country performance was superior to that of the M3 GMC and road performance exceeded that of the M10 GMC.
• Vehicle stability and handling were unchanged despite the larger gun.
• Braking and accelerating capabilities were excellent.
• The vehicle weighed 15 tons (13.6 tonnes), allowing it to use bridges and roads that the 30.5 ton (27.7 tonne) M10 GMC could not.

However, many shortcomings of the vehicle were also identified:

• The limited turret ring size decreased crew comfort and resulted in crew injuries when driving over rough terrain or when making short, jerky movements.
• The driver had limited vision.
• The vehicle had a negligible advantage in cross-country performance compared to the M10 GMC and no superiority in road performance compared to the M3 GMC.
• Only 44 rounds of 75 mm ammunition could be carried, which was considered inadequate.
• Due to the fragility of the traverse mechanism, the gun had to be fixed over the rear of the hull when on the move. This decreased the vehicle’s responsiveness when entering combat.
• The engines required 80 octane fuel, a fuel type not used by the M10 GMC or M3 GMC. This complicated logistics.
• Most notably, the vehicle was generally inferior to the 76 mm Gun Motor Carriage T70 (M18), production of which was on track to begin within a few months.

	“M8 GMC” (1943)	M3 GMC (1941)	M10 GMC (1942)	M18 GMC (1943)
Weight	15.1 tons (13.7 tonnes)	10.0 tons (9.07 tonnes)	33.0 tons (29.9 tonnes)	18.78 tons (17.04 tonnes)
Engine & Horsepower	2x Cadillac Series 42 (220 hp)	White 160 (147 hp)	GM Series 71 (375 hp)	Continental R-975 C1 (350 hp)
Power/Weight Ratio	14.56 hp/ton (16.06 hp/tonne)	14.70 hp/ton (16.21 hp/tonne)	11.36 hp/ton (12.54 hp/tonne)	18.64 hp/ton (20.54 hp/tonne)
Max Speed	46 mph (74 km/h)	45 mph (72 km/h)	30 mph (48 km/h)	45 mph (72 km/h)
Fuel Capacity	178 gal (674 L)	60 gal (227 L)	167 gal (632 L)	165 gal (625 L)
Cruising Range	175 mi (282 km)	200 mi (322 km)	140 mi (225 km)	100 mi (161 km)
Suspension	Vertical volute spring	Leaf spring (front) Vertical volute spring (rear)	Vertical volute spring	Torsion bar
Height	90.5 in (2.30 m)	98.625 in (2.51 m)	97.5 in (2.48 m)	93.25 in (sans .50 cal) (2.37 m)
Main Armament	75 mm Gun M3	75 mm Gun M1897A4	3 in Gun M7	76 mm Gun M1A1
Ammo Stowage (Main Gun)	44 rounds	59 rounds	54 rounds	45 rounds
Horizontal Traverse Limits	±180°	-19° / +21°	±180°	±180°
Turret Ring Size	54.4 in (138 cm)	n/a	69 in (175 cm)	69 in (175 cm)
Frontal Armor	1.125 – 1.75 in (28.6 – 44.5 mm)	0.5 – 0.625 in (12.7 – 15.9 mm)	1.5 – 2.25 in (38.1 – 57.2 mm)	0.5 – 0.75 in (12.7 – 19.1 mm)

*Data from afvdb.50megs.com and Can Openers

The Tank Destroyer Board concluded that the ‘M8 GMC’ was not fit for service, even as an expedient tank destroyer. By the time the vehicle’s many problems could be corrected and mass production could begin, the TD Board reasoned, the far superior T70 (M18) GMC would be ready for production.

Armored Force Testing

The Tank Destroyer Board was not the only group of evaluators to get their hands on the modified M8. The Armored Force also got their hands on the vehicle and performed their own tests to determine if the vehicle was suitable for service. Additionally, they also examined if there was a field requirement for such a vehicle. Unlike Aberdeen and the Tank Destroyer Board, the Armored Force did not give the vehicle a shortened name.

The Armored Force was able to compile a similar list of the modified M8’s faults:

• Crew size was decreased from 5 to 4 when compared to a standard M8 HMC.
• Firing, loading, and traversing the gun was made unnecessarily difficult as a result of poor ergonomics and mechanical failure.
• The M8’s 75 mm gun M3 was not as accurate as the Medium Tank M4’s. This was due to the M8’s usage of a modified howitzer gun mount.
• The turret was extremely off-balance.
• Internal stowage facilities were disappointing.
• No roof protection or anti-air capabilities.

The Armored Force bluntly concluded their assessment of the vehicle by stating that, as a result of multiple severe deficiencies, they considered the ‘M8 GMC’ to be entirely unfit for service.

However, as mentioned earlier, the Armored Force also sought to determine if there was any purpose to such a design. They reasoned that a light tank armed with a high-velocity 75 mm gun would be a vital asset, offering accurate and powerful fire from a maneuverable platform. As a result, they requested that the development and production of the Light Tank T24 (M24) be accelerated. The T24 was, of course, yet another light tank project to mount a 75 mm gun in the turret of a light tank. However, unlike its forefathers, the T24 would become a successful design, remaining firmly in the weight class of a light tank.

Coming to the same conclusion as the Tank Destroyer Board, the Armored Force requested that all development and consideration of the 75 mm Gun M3 on 75 mm Howitzer Motor Carriage M8 Chassis be halted. Faced with two very negative reports, the Ordinance Committee was left with a very easy decision. In June 1943, they requested that the project be scrapped. Later that same month, the request was approved. The modified M8 was sent back to Aberdeen Proving Ground and converted back into a regular M8 HMC.

Design

Firepower

Unsurprisingly, considering its extended designation, this vehicle was armed solely with the 75 mm gun M3. No machine guns were provided for anti-infantry or anti-aircraft duties. An unfortunate result of the modification process was that the gun breech no longer operated semi-automatically, decreasing the gun’s rate of fire. Another consequence of the expedient conversion was a comparative loss in accuracy. This was due to the use of a modified howitzer gun mount instead of a dedicated anti-tank gun mount. Firing tests between the modified M8 and an M4 ‘Sherman’ revealed that the M8’s accuracy was directly inferior to the M4’s.

The M3 gun could fire M72 Armor Piercing, M61 Armor Piercing Capped, M48 High Explosive, and M64 White Phosphorus rounds. The vehicle could carry up to 44 rounds of ammunition, slightly less than a standard M8 HMC’s 46 rounds of stowage.

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The 75 mm gun was a significant upgrade over the 37 mm gun because it performed better against both soft and hard targets. Firing M61 APC rounds, the 75 mm M3 gun could penetrate a maximum of 2.4 in (60 mm) of rolled homogeneous armor (RHA) at a range of 500 yds (457 m) and a 30º angle. In comparison, the 37 mm M6 gun, firing its equivalent M51 APC round, could only penetrate 2.1 in (53 mm) of RHA under the same conditions. It should also be noted that the 75 mm armor-piercing shell kept more of its penetration at range and performed better against angled armor when compared to the 37 mm armor-piercing shell. Additionally, the 75 mm gun had an extremely effective HE round. The 75 mm M48 HE shell had a hefty 1.49 lbs (676 g) of TNT filler, while the equivalent 37 mm M63 HE shell had just 0.085 lbs (39 g) of TNT filler.

	Armor-Piercing				High-Explosive
	37 mm M51	75 mm M67			37 mm M63	75 mm M48
Penetration @ 30° & 500 yd (457 m)	2.1 in (53 mm)	2.4 in (60 mm)		Explosive Filler	0.085 lbs (38.6 g)	1.49 lbs (676 g)
Velocity	2,900 ft/s (884 m/s)	2,030 ft/s (619 m/s)		Velocity	2,600 ft/s (792 m/s)	1,520 ft/s (463 m/s)
Projectile Weight	1.92 lbs (0.87 kg)	14.96 lbs (6.79 kg)		Projectile Weight	1.61 lbs (0.73 kg)	14.7 lbs (6.67 kg)

Compared to the 37 mm guns of the M2, M3, and M5 light tanks, the 75 mm gun of the ‘M8 GMC’ had superior armor penetration capabilities and a much more powerful HE shell. With its powerful gun, the ‘M8 GMC’ would have been able to engage and destroy both armored and soft targets with ease.

Protection

Armor-wise, the vehicle was essentially unmodified. It retained the same armor profile as a standard M8 HMC. The only small armor modification was made to the frontal turret armor. A hole had to be cut into the front of the turret to make room for the larger 75 mm M3 gun. When viewing the vehicle from the front, this created an extremely small area behind the upper part of the gun mantlet that had no additional turret armor behind it. However, this minute weakness was so inconsequential that, for all intents and purposes, the modified M8’s armor was left untouched.

Mobility

Similarly, the mobility of the ‘M8 GMC’ was identical to that of a standard M8 HMC. Both vehicles shared the same vertical volute spring suspension (VVSS), ‘Hydramatic’ automatic transmission, and twin Cadillac Series 42 gasoline engines which produced a net 220 hp @ 3,400 rpm. Fuel capacity and cruising range were 178 gal (674 l) and 175 mi (282 km), respectively. Despite the added weight of the larger gun, the ‘M8 GMC’ had impressive mobility, with the vehicle performing well both on and off road.

Crew and Ergonomics

For the modified M8, crew comfort and quantity were a point of contention. Standard M8 HMCs had a crew of four. The commander and gunner sat in the turret while the driver and assistant driver were in the hull. The ‘M8 GMC’ was no exception, with four seats for the four crewmembers. Both the Armored Force Board and the Tank Destroyer Board agreed that the vehicle’s two-man turret was a serious tactical shortcoming. Requiring the commander to load the gun in addition to his commanding duties was too overwhelming, they reasoned. However, while the Armored Force Board was content to test the vehicle as-is, the Tank Destroyer Board attempted to remedy this issue by adding an extra crewmember. Adding a dedicated loader to the turret would increase the vehicle’s effectiveness in combat, they argued. Executing this plan, however, was much easier said than done.

Multiple changes were made to the vehicle in an attempt to squeeze in an extra crewmember. The gunner assumed a standing position made possible by removing parts of the hull floor. However, his arrangement was rather uncomfortable due to the close proximity of the traversing handwheels and recoil guard. Additionally, the gunner could only traverse the gun 90° to the left and 45° to the right. At first, the Tank Destroyer Board attempted to position the commander to the left of the gun, as seen in the above left photograph. However, this crowded the loader and interfered with his duties. The commander was moved to his ‘traditional’ location behind the gunner, which gave the loader enough space to effectively load the gun. At the end of their report, the Tank Destroyer Board concluded that placing three crewmembers within the turret of the ‘M8 GMC’ would inevitably have a negative impact on crew safety and efficiency.

Conclusion

The ‘M8 GMC,’ ‘M8A1,’ or even the long-winded 75 mm Gun M3 on 75 mm Howitzer Motor Carriage M8 Chassis, was a failure. Although the vehicle performed quite well during its initial trials, numerous serious flaws were eventually discovered. A two-man turret, mediocre accuracy, unsatisfactory ammunition stowage, poor crew ergonomics, and, most importantly, general inferiority plagued the vehicle. Taking the time to perfect a flawed, expedient tank destroyer when a superior light tank, the M24 ‘Chaffee,’ and a superior tank destroyer, M18 ‘Hellcat,’ were well into development would be wasteful. Although promising on paper, the ‘M8 GMC’ simply had no role to fill.

75 mm Gun M3 on 75 mm Howitzer Motor Carriage M8 Chassis Specifications
Dimensions (L x W x approx. H)	14’ 6.75” x 7’ 4.5” x 7’ 6.5” 4.44 x 2.25 x 2.30 m
Weight	15.1 tons 13.7 tonnes
Armament	75 mm Gun M3 (44 rounds)
Armor	Turret Front: 38 mm Gun shield: 38 mm Side: 25 mm Rear: 25 mm Hull Front: 28.6-44.5 mm Side: 25-28.6 mm Rear: 25 mm Roof: 13 mm Floor: 9.5-13 mm
Crew	4 (driver, assistant driver, gunner, commander) or 5 (driver, assistant driver, loader, gunner, commander)
Propulsion	2x Cadillac Series 42, 220 hp @ 3,400 rpm, 14.57 hp/ton (16.06 hp/tonne)
Fuel Capacity	178 gal (809 l)
Top Speed	46 mph (74 km/h)
Cruising Range	175 mi (281.6 km)
Suspension	Vertical volute spring
Total Production	1 converted

Sources

Can Openers – The Development of American Anti-Tank Gun Motor Carriages by Nicholas Moran
Stuart – A History of the American Light Tank by R. P. Hunnicutt
Seek, Strike, and Destroy US Army Tank Destroyer Doctrine in World War II by Dr. Christopher R. Gabel
M3 & M5 Stuart Light Tank 1940-45 by Steven J. Zaloga
HMC M8: Quick Support for Light Tanks | Warspot.net
British Army Staff AFV Situation Report No. 1 July 18th 1942
TM 9-732B 75 mm Howitzer Motor Carriage M8 January 1944
75mm HMC M8
75mm GMC M3
3″ GMC M10
76mm GMC M18

United States of America (1940)
Mobile Pillbox – None Built

The early 20th century was dominated by new technologies being developed in large numbers. To capitalize on these rapid advancements, monthly magazines were published that focused on bringing these new technologies to the general public’s attention. This proved to be a great success. The most popular example of these magazines is Popular Mechanics, which published its first issue in 1902 and continues to be published today. Another popular example was Modern Mechanix, which went through several name changes since its first issue in 1928 before its final issue in 2001.

The technologies featured in these magazines varied greatly in their application. Power sources, home gadgets, farming equipment and flying machines are but a few examples of the kinds of inventions and concepts featured. Most notably, particularly during both World Wars, was the inclusion of conceptual weaponry and armored vehicles. These were rarely competently designed. Due to a total lack of practical insight into the use of military equipment, the end result was often a design more appropriate for a science fiction setting than a real battlefield. Some designs featured in these magazines are notable for their relative practicality however, at least when compared to the rest, and their intended usage is somewhat reasonable for being designed by illustrators as nothing more than magazine filler.

Firepower Required

Before the United States entered the War in 1941, it faced a distinct lack of dedicated tank destroyers. While it would not be until late 1941 when the US finally adopted such a vehicle – the 75 mm gun-armed M3 Gun Motor Carriage – designs already existed in the previously mentioned magazines that were intended to fill a similar role.

The November 1940 issue of Modern Mechanix features a drawing of a large armored truck with two guns in an even larger turret-mounted behind the cab. This Mobile Pill-Box Fortress, as it is referred to in the magazine, by virtue of having a single turret on a sensible and presumably existing truck chassis, is on the higher end of practicality regarding conceptual designs found in these magazines. No other name is given to the vehicle and no further information on it can be found despite supposedly being based on a prototype built by a truck manufacturer based in Los Angeles, California.

Design

The Mobile Pill-Box Fortress is based on a large truck chassis with two single wheels at the front and two pairs of triple wheels at the rear. The reason for two pairs of triple rear wheels should be clear, as directly above them is a huge domed turret housing a pair of 6 inch (152 mm) guns, presumably naval in origin.

Turret

The turret can rotate a full 360 degrees, but gun elevation and depression are not known. Depression would inevitably be limited in the forward arc due to the roof of the cab and the bizarrely located headlight mounted to it. Ammunition for the guns is stored in two racks, one upper and one lower. The shells are stored nose-up in two racks that run the full circumference of the interior turret wall. This allows a large number of projectiles to be stowed despite their great size. It is not shown in the drawing where the propellant charges are stored. It is possible they are stored at the front of the turret or on the right side of the guns where they would be obscured, but the most likely explanation is either that they were never considered by the artist or the shells are one-piece. No access hatch or door is visible on the turret.

Due to the great recoil generated by such large guns, the vehicle features four large outriggers around the turret ring. These outriggers appear to be telescopic in extension and fixed in place with no articulation, apart from being capable of extending and retracting their feet up and down. The outriggers are an appropriate design choice for a vehicle that, as the name suggests, acts as a stationary pillbox instead of a more mobile vehicle, capable of quickly relocating during combat.

The turret’s gunner is located on the left side of the guns and has no seat. He has a direct vision telescope that is mounted unusually far back in the turret which is aiming through a thin visor in the turret’s mantlet. Even though the sight would most likely move with the mantlet, and stay lined up to see through it, the field of vision as a result of being mounted so far back would be incredibly narrow. Only two other crewmen are shown in the turret, those being the loaders, who are each loading their respective guns. As 6 inch guns, each projectile would have been very substantial, at likely 45 kg (99 lb) or more in weight. With the turret having a pair of guns, this means that each loader has to lift and load projectiles by himself, which during sustained fire would be incredibly tiring without any loading aides such as a winch or conveyor, neither of which are shown.

Cab

The cab is located at the front of the vehicle. The driver’s position is assumed to be on the right side due to the placement of the only seat visible in the drawing, an unusual choice for an American vehicle. However, due to the perspective of the drawing, the seat may actually be more centered in the cab. On the left side of the cab is the assistant driver who operates at least one of the two machine guns present in the vehicle, both of which are in the front corners of the cab. Ammunition for the machine guns is stowed above the engine in the center of the cab. Due to the placement of the driver, it is likely that he operates the right-side machine gun instead of the assistant driver having to move back and forth between the two guns. Like some tanks with an assistant driver, it is likely that he would be expected to take over driving the vehicle should the driver be injured. They may also alternate duties each day.

There are a number of vision ports around the cab. There are two ports on the front slope which can be hinged open. Similarly, there is a large hinged port on the sloped roof. It can be assumed there is a second port on the right side which is obscured, but what these upwards-facing ports would be for is not clear. Each of the two machine guns in the front corners had their own fixed vision ports above them, which, like that on the turret, would provide undoubtedly poor visibility for those operating the guns. There is a fixed port on the left side of the cab, again it is likely the right side has the same. Lastly, there is a vision port in what appears to be an access door in the back left corner of the cab. A step is present below it on the outside, as is a handle. What appears to be two hinges spanning the width of the cab roof are also present. It is not clear how these panels would open.

Armor

No specific armor values for the vehicle are given, but while the drawing is poorly scaled it is clear that the armor of the turret is supposed to be very thick by standards of the time. The turret armor is intended to protect against shells and bombs (no specific shell or bomb is described), whereas no such requirement is given for the cab armor, but it is reasonable to assume it would be at least capable of resisting small arms and shrapnel. The engine has its own armored housing within the cab, and it is not known if the covers over the wheels are simply mudguards or if they too are supposed to be armored.

Fate And Conclusion

While at its core the Mobile Pill-Box Fortress is reasonably designed by the standards of the magazine it was featured in, no information can be found regarding the claim that it was based on a real prototype that underwent four months of testing by the US Army. After the United States joined the war, a great deal of effort went into developing and testing trucks carrying anti-tank guns in a wide variety of configurations.

The purpose of these vehicles was to be fast and easy to manufacture due to being built on existing chassis, as well as fast on the battlefield, able to quickly respond to reports of enemy tanks in an area and move to engage them. This manufacturing and doctrinal need are incredibly similar to the description of the Mobile Pill-Box Fortress, a truck-based vehicle capable of traveling up to 65 mph (105 km/h) to any threatened area to counter both tanks and infantry, and afterward, relocate to any other area in need of anti-tank support. However, due to the great weight of the vehicle, it is reasonable to expect it to be incapable of reaching such high speeds outside of long straight roads.

The choice of a 6 inch gun would be questionable, let alone a pair of them. The incredible capability of such weapons against both tanks and infantry cannot be understated, especially for 1940, but their immense size and weight directly influences the size of the vehicle, which in turn condemns it as almost entirely impractical. For the vehicle’s time, it can be argued quite easily that no practical advantage comes with having such large weapons in a vehicle like this, simply because far smaller and lighter anti-tank guns already existed that were perfectly capable of defeating any tank of the period. At the very least it would be easy to invent a more sensible gun for the drawing.

Despite the similarities between the purpose of the Mobile Pill-Box Fortress and the actual tank destroyers the United States would come to use, the sheer unwieldiness and weight of the vehicle would undoubtedly restrict it to roads only, greatly limiting its application as a strategically mobile weapon. The design, like so many from these magazines, is a great example of theory detached from reality and it is no surprise that none were ever built – this vehicle was purely for the readers of the magazine rather than actual use.

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Representation of the ‘Mobile Pill-Box Fortress’ produced by the Author, Mr. C. Ryan, funded by our Patreon campaign.

Specifications
Crew	At least 5 (Driver, Assistant Driver, Gunner, Two Loaders)
Speed	65 mph (105 km/h)
Armament	Two 6 inch (152 mm) guns, Two machine guns

Source

Modern Mechanix, November 1940