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WW2 US Prototypes

75 mm Gun M3 on 75 mm Howitzer Motor Carriage M8 Chassis

 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.

The 75 mm Gun M3 on 75 mm Howitzer Motor Carriage M8 Chassis in January 1943 at Aberdeen Proving Ground. ‘M8A1’ was Aberdeen Proving Ground’s unofficial designation for the vehicle. Source: Can Openers

Small Gun Problems

A destroyed M3 ‘Stuart’ of the British 7th Armoured Division in North Africa. Source: rommelsriposte.com

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.

The third production M7 medium tank. The vehicle’s visual similarity to the later ‘Stuart’ light tanks and the M4 Sherman is apparent. Source: Wikimedia

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.

No mock-up of the Light Tank T21 was ever produced. However, the vehicle bore a general resemblance to the Medium Tank T20, the mock-up of which is pictured here. Source: Stuart

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.

A diagram of an M8 Howitzer Motor Carriage. The vehicle mounted a new howitzer-armed turret on the modified chassis of a M5 Light Tank. Source: lonesentry.com

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.

The 75 mm Gun M3 on 75 mm Howitzer Motor Carriage M8. The vehicle was never given a shorter official designation, so both the Tank Destroyer Board and the Armored Force Board used their own nicknames for it. Source: warspot.net

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.

A view inside the turret of a restored M8 HMC. Note the rear roof plate and .50 caliber machine gun, both of which were removed on the converted M8 to improve crew comfort. Source: author’s own

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.

The ‘M8A1’ at Aberdeen Proving Ground. Source: Can Openers

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.

From left to right, Gun Motor Carriages ‘M8’, M10, and M3. Source: Can Openers

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.

In this photograph, the ‘M8 GMC’ has its gun locked over the rear of the hull in its ‘travel’ configuration. This was identified as a flaw by the Tank Destroyer Board, as the vehicle would be slow to respond to a combat situation. Source: Can Openers

Armored Force Testing

The ‘Motor Carriage M8 with 75 mm Tank Gun, M3’ arrives at Fort Knox to be examined by the Armored Force Board after the Tank Destroyer Board had run their course. Source: warspot.net

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.

A 75 mm Gun M3. Pictured here with the Mount M34, this cannon was mounted to early M4 Shermans. Source: theshermantank.com

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.

A diagram of a standard M8 HMC’s ammunition stowage layout. Source: afvdb.50megs.com
>
A photograph of the bulkhead racks of the ‘M8 GMC.’ Source: Can Openers

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.


An overhead view of the vehicle’s turret. Note the hole behind the gun mantlet. Source: Can Openers

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.

A diagram of a standard M8 HMC showing most of its internal workings. Source: afvdb50megs.com

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.


The Tank Destroyer Board tried multiple three-man turret crew configurations. The two pictured above were considered unsatisfactory. Source: Can Openers

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.


A photograph of the gunner’s position. Source: Can Openers

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.


An illustration of the 75 mm Gun M3 on 75 mm Howitzer Motor Carriage M8 Chassis by Pavel Alexe. Original M8 ‘Scott’ illustration by Tank Encyclopedia’s own David Bocquelet.

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

3 replies on “75 mm Gun M3 on 75 mm Howitzer Motor Carriage M8 Chassis”

What is the purpose of the racks or slats on the side of the standard M8 turret? I think I have seen similar devices on M10s, but I have no idea what it is at all.

Late production M8 HMCs had special mountings on the side of their turrets to carry spare track links. One such example can be seen here. Let me know if you have any further questions!

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