Saturday, 6 April 2019

HMC M8: Quick Support for Light Tanks

The idea of a specialized SPG for supporting tank and infantry units came to the Americans in the early 1930s. However, attempts to build such a vehicle ended up with nothing. As a result, the US Army began WWII without an SPG. The HMC T30, built on the M3 halftrack, entered trials in January of 1942. The SPG was launched into production in February of that year. Even though a large number of these halftracks was built, 500 units in all, the Americans saw it as only a temporary solution. Requirements for an SPG with a 75 mm howitzer on a tank chassis were still not met.

From a casemate to a turret

The difficult situation with a light close support SPG was not unique in the American army. There were issues with development of all sorts of SPGs. The M3 chassis was an easy way out. It was also used to build a tank destroyer (GMC T12, later standardized as the 75 mm GMC M3) and a 105 mm howitzer (HMC T19). Both of these were also considered temporary solutions, but they still enjoyed lengthy combat careers.

An SPG on a tank chassis was preferable, but slow help is no help at all. For instance, a SPAAG on a tank chassis was only adopted by the army at the end of the war. More than 6000 half-track SPGs had been built by that point.

The first HMC T47 variant, a step towards the future HMC M8.

Work to create a closed top SPG on the Light Tank M3 chassis resulted in the HMC T18. However, there was no consensus regarding the need to have a closed top. This system had its issues, including poor ventilation of the fighting compartment. Future work on SPGs built on the Medium Tank M3 chassis exclusively produced open top vehicles.

Work on a light SPG began in December of 1941. There was another reason why an alternative to the HMC T18 was required. The Americans came to the conclusion that vertical front armour was a bad idea. Work on the reworked Light Tank M3A1E1 with a sloped front began in October of 1941. The SPG with a sloped front received the index HMC T41. The T41 and its relatives deserve a separate tale. Here, let us only mention that it initially used the Light Tank M3E2 (future Light Tank M5) chassis, but the designers later changed the chassis to the Light Tank M3.

The early turret was not roomy enough.

Moving the gun into the front plate, and therefore moving the center of gravity forward, had its drawbacks. The load on the front wheels increased, and the maneuverability of fire of such an SPG was low. A decision was made to install the howitzer into an open topped turret. This idea was tested in the HMC T47 model, built in April of 1942, nearly at the same time as the revised HMC T41. This SPG also used the Light Tank M3 as a chassis. The upper front plate consisted of two sloped parts. The sides were also sloped.

The improved turret with a bustle. The hull is composed of sloped plates, yet is different from the Light Tank M3A3.

The most noticeable change was the enlarged turret ring. Increasing the width of the turret ring to 1382 mm allows the turret to fit three men. However, the turret had to be changed again, since it was too small for three crewmen. A second variant of the turret was built, equipped with a bustle, which survived further revisions. Like the first variant, this turret had a polygonal shape.

A reworked HMC T47, this time based on the Light Tank M5 chassis.

The small turret still offered decent working conditions to the crew thanks to the redesign of the gun mount. Until this point, the 75 mm Pack Howitzer was installed into SPGs as is. This design was bulky, and there were issues with protection of the recoil elements.

The same turret from another side. This was almost the final version.

The issue was resolved by combining the Pack Howitzer barrel with the slightly modified M34 gun mount, which was used on the Medium Tank M4. The result was a light gun mount without the drawbacks of previous systems. The mount, named 75 mm Howitzer Mount M7, was covered with a large mantlet.

75 mm Howitzer Mount M7, production variant.

The change to the turret was the first metamorphosis of the HMC T47, but not the last. A decision was soon made to change the chassis to the Light Tank M5. Specialists from Cadillac joined the army’s designers. A new model of the SPG was complete by May 15th, 1942. The turret was still welded and largely assembled from flat plates, but the number of plates decreased, and the volume of the turret increased. The gun mantlet was enlarged. Like before, the gun mount offered an impressive range of -20 to +40 degrees. After an inspection, the Ordnance Committee standardized the HMC T47 as the Howitzer Motor Carriage M8. This happened before a prototype was even built. This was a rare case for American tank building, which only demonstrates how much the army needed this SPG.

A stable light platform

A contract was signed with Cadillac for HMC M8 production. A logical solution, since this company also produced the Light Tank M5. The first production HMC M8 was ready in September of 1942, and was directed to the Aberdeen Proving Grounds. The turret changed yet again. The sides became curved, which increased the size of the fighting compartment. The mantlet was cast. Bulges from the recoil mechanism covers were evened out, and the barrel was covered up with a massive shield. A Browning M2HB pintle mount was added to the roof of the turret.

The first production HMC M8. Aberdeen Proving Grounds, September 22nd, 1942.

Changes were made to the hull as well. The bow machinegun, present on the Light Tank M5, was removed. The enlarged turret ring meant that changes had to be made to the front of the hull. It was not possible to install hatches in the roof. Observation hatches were added in the front plate. These hatches were not large enough to enter and exit the vehicle. The driver and his assistant received periscopic observation devices in the driver’s compartment roof. 

The same vehicle from the rear. The AA machinegun mount is shown.

The layout of the fighting compartment was unusual. Like the Light Tank M5, the SPG had a crew of four. During travel, two of them were housed in the turret. In battle, they were joined by the assistant driver, who was also the gunner. Unlike in light tanks, where the gunner was placed on the left, the gunner was on the right side of the HMC M8. The turret also had neither a basket nor a hydraulic traverse mechanism, unlike the Light Tank M5. The loader often workes while standing, but did not feel as uncomfortable as the loader in the Light Tank M3. The drive shaft was low enough that he didn’t have to leap over it as the turret traversed.
Cutaway diagram of the MHC M8.

The creators of the HMC M8 seriously considered the comfort of working in the cramped fighting compartment. The level floor helped moving when the turret was traversing. The ammunition was laid out well. The 6 round ready rack was located in the front. 9 more were stored in the left sponson. Finally, the main ammunition rack containing 31 rounds was located in the rear of the fighting compartment. The right sponson contained the SCR 510 radio, operated by the commander.


The fighting compartment was rather roomy for a vehicle of this size.

Comparative trials performed in early 1943 between the HMC M8, GMC M10, and 75 mm GMC M3 showed that the M8 was very quick. The light SPG was faster than its competitors on dirt roads. In off-road conditions, the M8 and M10 were about even, both faster than the M3. On a highway, the HMC M8 drove at a speed of 48 kph. On a straightaway, the vehicle reached a speed of 73.5 kph, which was a little higher than the calculated top speed. It took 19 seconds to reach a speed of 48 kph. The drivers noted that the brakes worked well.

HMC M8, late type. The holders for track grousers can be seen on the roof.

However, the HMC M8’s crew had the worst conditions at high speeds. The odds of getting injured were high. The 1382 mm turret ring was deemed too small for three men. The driver’s visibility was worse than on the other vehicles.

Nevertheless, the overall impressions were good. The HMC M8 was a fast and stable platform for the 75 mm howitzer. The small mass of 15.7 tons allowed it to cross bridges that were impassable for the GMC M10.

Like the Light Tank M5A1, the late HMC M8s received a storage box in the rear.

The new SPG was also used in the tank destroyer program. Available space in the fighting compartment and the use of the M34 gun mount hinted at the ability to install the M3 gun barrel. The result was an SPG sometimes known as the M8A1, but this index was never used in reality. However, by the time it was presented in January of 1943, the GMC T67 with a more powerful 76 mm gun already existed. The development of the GMC T70 was already launched as well, which would also feature the 76 mm gun.

The re-armed HMC M8, sometimes called M8A1.

Changes to the design compared to the production HMC M8 were minimal. The turret was slightly changed. The AA MG mount was removed. A cutaway was added to the front, and a counterweight to the rear. The gun mount didn’t change much, and only a more powerful recoil system was added to acommodate the more powerful gun. The M8A1 was tested in Fort Hood starting in February of 1943.

Trials showed that the new gun did not affect the characteristics of the vehicle much. At the same time, the turret was cramped for three people. Different crew positions were tested, but that did not help much.

The longer M3 gun fit without problems.

The converted SPG was sent to Fort Knox, where a second round of trials was carried out. By then it was clear that the HMC M8 tank destroyer will not enter service. Nevertheless, the vehicle was still of some interest. Work on the Light Tank T24, which was armed with a 75 mm gun, began in March of 1943. The M8A1 became a test lab for carrying the 75 mm gun on such a small chassis. The results were positive, but the gun mount for the Light Tank T24 had to be changed to reduce the recoil length. In addition, it was recommended that the turret ring be increased in diameter. The Light Tank M24 had a 1520 mm wide turret ring.

There were no issues with the ammo rack layout.

The first 197 HMC M8 were armed with the M2 howitzer. After that, a simplified version was used, standardized as the M3. The M7 gun mount was not changed, since the M2 and M3 were interchangeable.

One of the variants of arranging the crew. The changes made to the turret are visible.

The design of the HMC M8 did not change much during production. Most changes were connected to the base vehicle, the Light Tank M5 or M5A1. The most noticeable change was the introduction of mounts for track grousers on the sides of the turret. A storage box was installed on the rear of the hull around the same time. The SPG remained in production until January of 1944. 1778 units were built in total.

Shoulder to shoulder with light tanks

The first production HMC M8s were complete in September of 1942, but the combat debut of the SPGs was delayed. Unlike the GMC M10, these vehicles were not used in North Africa. The 1st Armored Division only received them in June of 1943, to replace the HMC T30. The doctrine of their application was composed at the same time: the M8 fought alongside the Light Tank M5 as close support vehicles. This combination was used in Sicily, and later in Italy.

HMC M8 in Italy, 1944. The front vehicle is towing a captured German ammunition cart.

Experience showed that the conclusions made as a result of trials were correct. Thanks to their small mass, the M8s were more agile than the M10s, which was an important advantage in the mountains. However, the Americans were too late with the creation of a 75 mm SPG. A more powerful gun was needed for support. The program to install a 105 mm M3 howitzer on the chassis of the Light Tank M5A1 was launched in November of 1943. Thid didn’t mean the end of the HMC M8’s career. A new SPG could not appear instantly, and the Light Tank M5A1 was deemed obsolete after the Italian campaign as well. There was nothing to replace the M8s with. It was impossible to wait until industry could produce new SPGs, and it was necessary to fight with what was at hand.

Loading an M8 onto a landing ship.

It’s hard to call the M8 a bad vehicle. A large range of gun elevation allowed it to carry out different kinds of firing missions. A 105 mm howitzer was not always necessary, and the 75 mm howitzer was often enough. The HMC M8 was also not defenseless against German tanks. In addition to M48 HE and M64 smoke shells, the M8 could fire M66 HEAT rounds. According to American data, these shells could penetrate 89-91 mm of armour. Of course, this was not among the SPG’s first priorities, but if necessary it could be used for self defense.

The HMC M8 was used widely in 1944-1945 in Western Europe

The M8 was also widely used in France in the summer of 1944. These SPGs were included in reconnaissance groups, largely supporting the same old Light Tank M5A1. The quick SPG was a good addition to the light tanks. It could defeat targets that were too tough for 37 mm guns, and the ability to lob shells in a steep arc came in handy. It is not surprising that the M8 was used in large numbers until the end of WWII. Even the introduction of the Light Tank M24 did not change the situation. This vehicle had its advantages when firing directly, but the M24 couldn’t fire indirectly like the M8 could.

These SPGs were often used to support infantry.

The M8 was also used in the Pacific. Here they proved themselves to be effective infantry support. The vehicles were equipped with snorkels during landings, which allowed them to land into shallow water. Crews often had to use the AA machinegun, but largely to fire at ground targets.

The M8’s turret was used on another vehicle, the amphibious LVT(A)4. The later LVT(A)5 used a modified version of this turret.

The M8 was also used in the Pacific.

The American army had 862 HMC M8s left at the end of WWII. Considering the intensity with which they were used, this is a fair amount of survivors. These SPGs were quickly removed from service after the war, partially replaced with the HMC M37 on the chassis of the Light Tank M24. They were never fully replaced, as the rate of armoured vehicle production radically decreased after the end of the war.

French vehicles served for a long time. An HMC M8 column in Indochina, early 1950s.

Officially, the American light SPGs were not distributed through Lend Lease. There were plans for this. The British army even assigned the name Scott to the M8, but the shipment didn’t happen. The M8 was not even considered for use in the Red Army.

Nevertheless, these SPG saw battle under a foreign flag. The French received SPGs directly from the Americans. In total, the “Fighting French” received 170 vehicles, used in reconnaissance units. After the end of WWII the French HMC M8 fought in Indochina.

Yemeni SPG in Aden, 1961

140 of these SPGs were also in use by the Italian army. The Italians received them from the Americans and kept using them until the end of the 50s. Yemen used these SPGs until the early 60s at least. The Mexican army later reused turrets from its M8s on the V-100 Commando armoured cars.

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