Once More on a New Chassis

Even though American self propelled artillery lay dormant for a long time, the Americans were the first nation to enter the war with an existing SPG. This was the GMC T12, built on a halftrack chassis. The halftrack SPGs were a necessary compromise, but they allowed to quickly saturate American units with motorized artillery. Production of analogous SPGs on fully tracked chassis began in 1942. This approach proved fruitful, and by the spring of 1943 the Americans not only caught up to the Germans in SPGs but took the lead in some aspects. However, there were areas where they never managed to catch up. This is mostly true for heavy self propelled guns, but this is the Americans' own fault. Their answer to German heavy SPGs only went into battle towards the end of the war in Europe and the GMC M40 was only standardized in May of 1945.

Better late than never

The main reason for the strange position that American heavy SPGs ended up in was the Field Artillery Command. It seemed that American artillerymen had not heard of the lessons drawn from experience in the First World War. The importance of mobility for heavy artillery was already clear by then. Even the presence of prime movers did not provide for satisfactory mobility. The conclusion was obvious: the only solution was self propelled guns that could move quicker than towed guns, deploy quicker, and also pack up and change positions quicker.

These conclusions could not be implemented into practice immediately for several reasons: sudden decrease in budget after the end of the war, "brain drain" in the American armed forces, and lack of suitable tank chassis. Work died down for 20 years, and after that the naive opinion that everything will be solved by high speed tractors reigned supreme. American Field Artillery Command did not stop to think that towed artillery has a number of limitations, including top speed (even artillery with rubber tires or steel wheels could not travel faster than 30-35 kph) and time spent setting up firing positions.

Deploying a 155 mm M1 gun to fire. It is interesting to think what would have happened if Field Artillery commanders who thought that tractors were a good enough solution were made to deploy the gun themselves.

This opinion also applied to the backbone of American heavy artillery: the 155 mm gun M1 or Long Tom. This gun was designed to replace the 155 mm GPF gun. Development lasted for nearly 20 years and it was only accepted into service in 1938. As often happened, this system was a duplex. The M1 carriage could also take a 203 mm howitzer. This solution was similar to the Soviet Br-2/B-4 guns, but while only a handful of Br-2 guns were built the American 155 mm gun was built in impressive numbers: 1882 units from October 1940 to June 1945. The gun had impressive characteristics and a good carriage that allowed it to be transported relatively quickly. However, there is no such thing as miracles. The gun weighed 13.8 tons in travel position, so mobility was not high. There were no plans to develop an SPG with the M1 gun.

GMC T83 pilot, summer 1944.

The situation did not change even after the GMC M12 was accepted into service in July of 1942. It was armed with the M1917 or M1918 gun, the same guns that the M1 was supposed to replace. It quickly turned out that the GMC M12 was much more mobile than towed guns, but Field Artillery Command was not swayed. They still considered the Mack NO truck and later the M4 High Speed Tractor to be sufficient for heavy artillery. The situation began to change only by early 1944 when it was clear that something heavy and mobile is going to come in handy when breaking through the Siegfried Line. A modernization program for the GMC M12 and the complementary Cargo Carrier M30 was put into effect. On March 9th, 1944, the Ordnance Committee recommended the development of a new SPG equipped with the M1A1 gun. The chassis also changed: now it was the Medium Tank M4.

The gun at maximum elevation.

The task was different than the one given for the development of the GMC T6. This time the requirements called for a mobile vehicle designed to support tank units. The concept was similar to what the Germans did with the Hummel, but with some differences. First, the vehicle was to be suitable both for direct and indirect fire from the start. Second, the source chassis could not be used without changes. The gun was quite large and even early calculations showed that the mass would be about 30,390 kg or comparable with that of the Medium Tank M4. This was not the limit, since the vehicle was going to be 3150 mm wide compared to the M4's 2620 mm. The result was a new chassis built with M4 components and a Continental R975 C4 engine as the most compact option. The Ordnance Committee authorized the start of work on this vehicle on March 23rd, 1944, giving it the name Gun Motor Carriage T83.

The fighting compartment was roomier than on the GMC M12. Ammunition racks for 20 shots were located behind the gun.

Development was given to the Tank-automotive Center in Detroit and the Pressed Steel Car Company in Pittsburg. This company also built the GMC M12, so the choice was obvious. The Medium Tank M4A1(76)W was in production by that point, but the chassis was unsuitable for this vehicle. It was clear that the tank's mass will only grow, and the failure of the Medium Tank T23 was still fresh in everyone's minds. The suspension of the GMC T83 was changed from the start. The HVSS suspension with 580 mm wide T66 metallic tracks was used.

An extra section of "floor" folded out in the rear.

The Ordnance Committee requested five prototypes. Five cargo carriers were also ordered to work in tandem, same as the M12 and M30. The Cargo Carrier T30 had the same chassis as the GMC T83 but carried 100 155 mm rounds rather than a gun. The assistant driver received a machine gun, but it offered him no protection while it was in use.

Second T83 pilot on trials.

This time the SPG and ammunition carrier appeared at the same time: on July 28th, 1944. The mass was much higher than expected: 37 tons. This was not a surprise. Even though the armour was reduced to 12.7 mm all around, the vehicle was 1280 mm longer, 530 mm wider, and 105 mm taller than the source tank (not counting the gun, with the M1 installed the vehicle was 3300 mm high). Thanks to the wide tracks, the ground pressure was only 0.77 kg/cm². The power to weight ratio at normal power was 10.8 hp/ton. These parameters were quite normal. Initial requirements called for 25 rounds of ammunition on board, but only 20 were carried. This was not a lot, but the GMC M12 only carried 10 and they were placed much less conveniently.

Cargo Carrier T30. Unlike its predecessor, it was not put into production.

The layout of the GMC T83 was similar to that of its predecessor. The gun was located in the rear part of the SPG. A large gun shield was installed. The idea of extra "floor" over the spade remained, but it was significantly altered. Now the floor slid out and had an extra section in the rear. The ammunition racks were placed behind the gun and they were now much easier to use. The gun crew of 6 (with a total crew in the vehicle of 8) was much better placed. All crewmen now had seats. Many improvements were implemented.

The first HMC T89. The chassis was the same as the GMC T83.

The result of this work was obvious after the first trials at the Aberdeen Proving Grounds. Trials of the GMC T83 showed that it was an even better gun platform than the M12. Trials were conducted both with a lowered and a raised spade to model the scenario where the gun had to open fire in a hurry (an important feature when assaulting the Siegfried Line). The SPG worked well in all conditions. The mobility was quite good. It could hold a speed of 39 kph for a short time, a decent result for such a heavy vehicle.

The first vehicle of this type was a conversion from the first GMC T83.

The gun fired 200 shots in the first stage of trials, after which a decision was made to replace the weapon with a 203 mm M1 howitzer. This replacement was not difficult to make. This conversion was not done haphazardly. The Ordnance Committee ordered work on the HMC T84 on the Medium Tank T26E1 chassis to begin in the spring of 1944, but the work dragged on since the base tank was far from finished. A logical idea to build a similar vehicle on the GMC T83 chassis was born. Since the ammunition racks were universal, this conversion was easy. The converted GMC T83 fired 75 shots. These trials showed that there is no problem with this conversion. The Ordnance Committee named the howitzer variant of this vehicle HMC T89 on November 2nd, 1944. The last 2 of the 5 pilot vehicles were built in this configuration in January of 1945.

The rear of the GMC T83 and HMC T89.

The second and third GMC T83 pilots built in October of 1944 went through intensive trials at Fort Bragg. A list of changes required of production vehicles was composed. One of the most important requirements was increasing the size of the fighting compartment. Correction of other defects was also required. Nevertheless, the vehicle was a clear success. Mass production was a done deal, with a few caveats. One was that the vehicle was not standardized before it went into production. The other was that the cargo carrier was not put into production at all. M4 tractors with M23 trailers would be used instead.

Good, but late

Production would take time to set up, so a decision was made in January of 1945 to send one GMC T83 and one HMC T89 prototype to Europe. The third pilot GMC T83 and one of the T89s were chosen. The T89 was modified to the level of a production vehicle, making all the changes noted after the trials. Battlefield trial were conducted as a part of Zebra Mission, the same operation that oversaw the first deployment of the Heavy Tank T26E3. The new SPGs were on the front lines by February of 1945 when Pressed Steel was just delivering its first production GMC T83s.

HMC T89 firing in the vicinity of Cologne. By this point it was rearmed with the 155 mm M1 gun.

The vehicles were allocated to the 991st Artillery Battalion that fought as a part of the 3rd Armored Division. The battalion was armed with the GMC M12, so the SPGs were put to good use. Field modifications began instantly. The howitzer was taken off the HMC T89 and replaced with a 155 mm M1 gun for the purposes of rationalization. This converted SPG was one of the first to fire on Cologne on February 27th, 1945. The battalion was often allocated to infantry divisions for support. Later it was reunited with the 3rd Armored. The HMC T89 got its howitzer back and it fought in its initial form. The SPGs were highly rated after their use in battle, which had an impact on production plans.

Production GMC M40.

Even though use in battle was limited to the Zebra Mission, this was enough to make a decision regarding standardization. The GMC T83 was standardized as the GMC M40 in May of 1945. Meanwhile, the vehicles produced at Pressed Steel transformed. For starters, they were equipped with 155 mm M2 guns that had a different breech. The use of new tracks was more noticeable. Practice showed that metallic T66 tracks chewed up rubber tires, and so later production vehicles used T80 rubber chevron tracks.

There were many small changes that separate the production vehicles and the pilots.

600 GMC M40 were ordered in total, a step forward compared to the 100 GMC M12. The fate of the HMC T89 was also decided as a result of battlefield trials. Pressed Steel received an order for 576 of these vehicles. Production began in June of 1945, but it took longer to become standardized. The end of the war changed things. Tank and SPG production was inevitably going to be cut. The US Army did not have an infinite budget, and so production volumes were reduced starting in the summer of 1945. SPGs had a lower priority and suffered the most. The fate of new SPGs was sealed when Japan capitulated, putting an end to the Second World War. Production of the GMC M40 ended in September of 1945 with 418 units completed. Only 48 HMC T89s were built. 24 GMC M40s were also converted to take a howitzer. The HMC T89 was standardized as the HMC M43 in November of 1945. Like the GMC M40, later models had T80 tracks.

HMC M43 as seen from above.

The short trip to Europe resulted in a large list of improvements, many of which were experimental. Direct fire missions, a common occurrence for the GMC M12 and M40, resulted in frequent clashes with enemy infantry. The armour of these SPGs was thick enough to withstand rifle fire, but there were losses from shell splinters coming from the sides and rear. The GMC M10 and M36 also had similar problems which were solved by introduction of roofs. 

A GMC M40 with a removable roof. The mass and size of this solution were excessive.

The development of such a roof was ordered by General Barnes himself, who personally headed the Zebra Mission. The project was carried out by staff of the Aberdeen Proving Grounds who produced a 1:8 scale model. The result resembled a barn, both visually and in size. The roof covered the fighting compartment from the sides and rear, but it greatly increased the dimensions of the vehicle. Nevertheless, orders were given to produce a full scale model.

The idea of auxiliary machine gun armament for self defense was developed in parallel. Four SPG mounts were added.

The second GMC T83 prototype, also called M40 by this time, was used. A standard Browning M1919A4 bow gun mount was also installed. The roof was made from sections on hinges and could be folded up if necessary. Rather than arguing with the general, proving grounds staff simply calculated the mass for the required thickness and dimensions of parts. Either an aluminium alloy or steel would be used. The lightest variant of the roof weighed 1429 tons and the heaviest weighed 3.5 tons. The idea was dropped.

A version with a recoilless rifle. This idea was also dropped as there was a risk of burning the crew with the exhaust gases.

Another requirement was increased self defense firepower. The bow gun alone was not enough. A GMC M40 with armament that any technical would be jealous of entered trials in June of 1945. Ball mounts for Browning M1919A4 machine guns were added to the left and right on the front of the fighting compartment, and two more machine guns on pintle mounts pointed backwards. As an alternative, 57 mm T15E13 recoilless rifles could be used instead of the front machine guns. A third alternative was one 75 mm T21 recoilless rifle. This solution offered all-round fire, but raised a lot of difficult questions. There was no need to build a gun truck out of this SPG since its job was not to engage enemy infantry. The installation of recoilless rifles was a bad idea, since the exhaust gases could injure the crew. This idea did no progress past the experimental stage.

Mortar Motor Carriage T94 with a dummy mortar. It was never built in metal due to decreased interest.

The SPG duplex could have been a triplex. The Ordnance Committee approved the development of the Mortar Motor Carriage T94 on February 22nd, 1945. The 254 mm T5E2 breech loading mortar was used as the armament. The T6E2 mortar of the same caliber was also proposed. Since the mount was the same as the 155 mm Long Tom, there was no problem with this plan. The estimated mass of the MMC T94 was 36,287 kg. The ammunition capacity was the same as of the GMC M40: 20 rounds.

Later production GMC M40s had M2 guns and T80 tracks.

The design was a good one, but did not progress past a full scale model. A crane for loading the rounds was installed on the right side of the casemate. The crew was reduced to 6 men. The vehicle was renamed 250 mm Mortar Carriage T94 on May 2nd, 1946, but this was more of a formality. Interest in this vehicle waned and work stopped.

The GMC M40 was widely used in the Korean War.

Unlike the GMC M12 that was quickly written off, its replacement had a different fate. The superior and more numerous vehicle remained in service for much longer. It was also modernized, in part, T84 tracks were installed. The M40 and M43 fought in Korea like this. These were the most powerful SPGs used in that war.

The HMC M43 also made it there. It was the most powerful SPG used in Korea.

The GMC M40 and HMC M43 remained in service into the late 1950s. In 1950 a number of these SPGs were sent to France as military aid, where they served until the late 1960s. The British were the second most numerous foreign user with over two dozen of these vehicles. Some say that it was named M40 Cardinal in the British army, but that is not the case. The name Cardinal applied to a different vehicle, the HMC M44. The M40's designation was unchanged. They remained in use by the BAOR until at least the mid-1960s. The M40/M43 were lucky and about 10 of them survive to this day, 2 of which were formerly in British service.

A partial solution to the issue of self defense. Infantry hitched a ride on any available vehicle.