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Big Self Propelled Caliber

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Unlike SPGs armed with light or medium caliber guns, the British and Americans had a harder time with heavier weapons. This is strange, since SPGs were initially envisioned to mechanize heavy artillery. The British stalled completely in this direction and work did not progress past several prototypes. The Americans fared a little better. The first production vehicles appeared in the fall of 1942 and went into action in the summer of 1944. This was the Gun Motor Carriage M12, the first American SPG with a heavy gun. This article will cover this vehicle's complex fate.

Carrier for the GPF

The French 155 mm Canon de 155 Grande Puissance Filloux (GPF) mle.1917 gun was one of the best heavy guns of the First World War. It was developed at APX (Atelier de construction de Puteaux), the largest French artillery manufacturer, by Colonel Louis Filloux. The gun was very successful, but also very heavy, weighing 13 tons in travel position. The French purchased any and all tractors they could get their hands on to move these guns. The Americans also used these guns under the index M1917 and put their own licensed copy into production under the index M1918.

The Americans were also faced with the issue of transporting these systems. The solution was simple: mechanization. The Americans were not the first to come up with the idea, as Louis Filloux already proposed SPGs for heavier guns, for instance, the 194 mm Canon de 194 mle GPF. The carriage for this weapon built at FAMH by Émile Rimailho was rather strange. The vehicle as made up of two parts: a chassis with the gun and a munitions carrier that also housed the engine and generator. The electric motors that powered the vehicle were installed in the gun chassis. This was a complex design, but a functional one. The Americans wanted something simpler, but also ended up with a strange design, or rather designs. The most successful was the Christie M1920 155-mm Gun Motor Carriage designed by John Walter Christie. This was the best SPG of its class at the moment of creation, but it was too expensive. Another nail in the coffin was that the American military could no longer agree on what it wanted.

Christie M1920 155-mm Gun Motor Carriage, the first successful American 155 mm SPG.

Work on heavy SPGs froze for 20 years. There were several reasons for this. One was that there was no money for this kind of work. Another was that there was no appropriate chassis. A medium tank was needed to carry this kind of gun and the Americans were in a rut when it came to those. As such, the artillery branch had to stay with towed guns. The light at the end of the tunnel came after the start of the war. The Medium Tank M3 program was launched in the summer of 1940. This vehicle was suitable as a chassis for heavy SPGs. One problem was that the tank was nothing but a set of requirements and plywood models until March of 1941. Work only resumed when mass production of the new tank began at the Detroit Tank Arsenal.

GMC T6 prototype, February 4th, 1942. The exhaust pipe is sticking out of the side of the chassis.

The first signs of revival were seen in June of 1941. This time, the work was triggered by the Ordnance Department, who remembered the First World War well. The artillery branch still considered towed guns sufficient so long as they had powerful artillery tractors and plenty of them. This concept won in the end. The M4 High Speed Tractor developed at Allis-Chalmers with elements of the Light Tank M3 appeared in 1942. This idea had its drawbacks, namely the length of time it took to set up the gun to fire, but also a significant advantage. The tractors were simple, cheap, and could be produced in great numbers. The artillery branch did not show a particular interest in an SPG with a 155 mm gun.

The gun is locked in travel position.

Despite this situation, the Ordnance Department still authorized the development of an SPG with the 155 mm M1918A1 gun on the chassis of the Medium Tank M3. The vehicle would be developed within the Department as well as at the Rock Island Arsenal. The task was not a trivial one. The Medium Tank M3 chassis was not big enough for the nearly 6 meter long gun. The only possible option was the same one as the SPG's predecessor: a self propelled gun mount with a fighting compartment in the rear. The Medium Tank M3 chassis had to be seriously rearranged.

The vehicle is in fighting position showing the maximum gun depression.

The new vehicle was indexed 155 mm Gun Motor Carriage T6. It was clear that it would be a peculiar one from the start. In addition to banishing most of the crew to the outside, the amount of ammunition carried onboard was quite low. The idea of a separate munitions carrier was an obvious one. Unlike the French, the two vehicles were completely independent. The munitions carrier was built on a similar chassis. The Germans had the exact same idea when building the Hummel

The SPG was quite tall, but the silhouette was lowered due to the lack of a casemate.

The experimental GMC T6 prototype was ready by early February of 1942. The running gear remained the same as the M3, but everything above the panniers was changed. The engine was moved to the center to free up space in the rear fighting compartment. The exhaust pipes now stuck out of the sides. This made access to the engine more difficult, although in many similar SPGs it was also necessary to disassemble half of the vehicle to get to the engine. Here it was only necessary to take off the gun. After that, it was easier to access the engine than on the Medium Tank M3. Even the aforementioned Hummel needed much more work. Fuel tanks were placed in the panniers.

The fighting compartment floor of the prototype could extend backwards. This proved to not be the best idea.

The crew consisted of 6 men, two in the driver's compartment and the rest in the fighting compartment. Thanks to the new engine placement, the driver's compartment was quite tightly packed. The long barrel hanging over the front required a new arrangement of hatches. The driver and his assistant had two hatches each, one above and one on the sides. Since the side hatches were only to be used when the gun was hanging over the main hatch, they were not very comfortable. Observation hatches like on the Medium Tank M3 were also added to the front. As for the other crewmen, they sat in the fighting compartment. The ammunition was housed under their seats and next to them. The ammunition capacity was not high, just 10 shots. Due to the design of the vehicle it was more comfortable to load the gun while standing on the ground anyway.

Most complaints recorded during trials had to do with the spade. It was changed most of all.

One of the drawbacks of the GMC T6 was the high bore axis, 2200 mm. The overall vehicle was 2883 mm tall. Other vehicles in the same class were also quite tall. The T6 had a good reason for its height. At maximum elevation it was necessary to dig a pit behind the gun for the GPF to recoil into. The same was necessary for the first American SPGs. There was no need to do this on the GMC T6. The vehicle was built with experience gained during the design of other such SPGs in mind, including the fact that the recoil during firing was high. The GMC T6 had a large spade attached to the back. It had a hydraulic drive that could raise and lower it.

The first stage of GMC M12 prototype trials at the Aberdeen Proving Grounds.

The new SPG arrived at the Aberdeen Proving Grounds on February 12th, 1942. Trials showed that the idea with the chassis was a good one and it was stable when firing. A part of the crew exited the vehicle and worked on the ground. One big issue was revealed during the trials: the spade. It was located too low and clipped terrain features when driving off road. During firing it also got in the way of the loaders. The hydraulic drive was susceptible to shock during firing and broke periodically. The spade had to be redone.

The SPG showed itself to be a stable platform but needed a new spade.

The SPG was sent for conversion and then to Fort Bragg, the home of American Field Artillery Command, who considered the towed guns to be sufficient. The generals were forced to reconsider their position. The mobility of the GMC T6 significantly exceeded the towed gun. It took just 35 minutes for the SPG to relocate to a position 9.6 km away, but it took a towed M1918A1 3 hours. The vehicle was a success. On July 17th, 1942, Field Artillery Command recommended accepting the GMC T6 into service after resolving a small list of defects. Soon after the vehicle was standardized as the 155 mm Gun Motor Carriage M12 (GMC M12).

Conservative mass production

The fact that the GMC M12 was standardized did not mean that field artillery was convinced. The initial order was made for only 50 vehicles and increased to 100 on August 12th. This was it. Despite the obvious superiority of the SPG over towed guns, high speed tractors were still the priority. This kind of conservatism was commonplace in American artillery.

23rd GMC M12 on trials, November 1942. The right side of the vehicle is very different from the prototype.

Since the decision to produce the GMC M12 was made in mid-July of 1942, production was unlikely to start before the fall. This is when the situation became interesting. The chassis was based on the Medium Tank M3, which was no longer in production. The issue of finding a factory to build these SPGs was also a tough one. Pressed Steel Car Company from Pittsburg received the contract. Interestingly enough, PSCC did not build the Medium Tank M3 as such. From August 1941 to July of 1942 they produced the Grant I, the British variant of the Medium Tank M3. PSCC was also the first factory where production of the Medium Tank M4A1 began. The first vehicles of this type, still resembling the Medium Tank T6, were built in March of 1942. The use of the M3 chassis for production of the M12 had some nuances.

35th GMC M12. Initially the exhaust system was not much different than the one on the GMC T6, but later it was changed based on experience in training.

The GMC T6 was sent to Pressed Steel where it served as a sample for production. The first production vehicle was ready in September of 1942. There were few differences from the prototype, but the chassis was clearly from the Medium Tank M4A1. The pannier next to the assistant driver was lengthened, which ate up his side hatch. The exhaust system was partially moved to the panniers. The spade was changed once again. There were a number of changes made to the fighting compartment. The gunner received a folding armoured plate to protect his head. A ventilation fan was added to the driver's compartment, a necessary feature given the proximity of the engine. The guns were not made anew, instead existing M1917 and M1918 guns were used. There was no need to renew production of the GPF given that the more powerful M1 gun was already available. This solution also used up existing stocks of the older gun.

The fighting compartment was made more comfortable, in part thanks to the new spade.

Production was not hurried. Pressed Steel produced 37 vehicles in October, but about 10-15 in subsequent months. 60 vehicles of this type were delivered in 1942, 40 more in 1943. The last vehicles were delivered in March. Field Artillery Command was not bothered with deployment, as these vehicles were more of an academic interest to them. The SPGs were put through lengthy trials in order to develop requirements for heavy SPGs. This is yet another illustration of American artillery commanders attitude regarding self propelled artillery.

Rear top view showing the ammunition racks and steps on the spade that made it easier to load the gun.

This was a strange decision, but the artillery branch had its reasons. A year of trials led to a list of requirements. These were not field modifications, but rather a thorough conversion. There were complaints about the exhaust system, the spade, and other components. A modernization was needed. There were plans to modernize 75 vehicles, but in the end only 74 were modified.

An SPG modernized at Baldwin Locomotive Works. Modernized vehicles could be distinct from one another.

The modernization order was prepared in December of 1943. This time Baldwin Locomotive Works was given the contract. Work lasted from February to May of 1944, which resulted in changes to the vehicle. The ammunition racks changed and became more comfortable to use. A radio set was installed. The spade change once again. New steps were added (they existed from the start on the M12, but were hard to use), which made it easier to load the gun. A number of vehicle received extra mud flaps and supports for a tarp. The effectiveness of the vehicle and its reliability increased slightly.

Cargo Carrier T14. These vehicles worked in tandem with the SPG. This is what they looked like initially.

As mentioned above, the idea to create an ammunition carrier came up while developing the GMC M6. Since the SPG was a priority, the ammunition carrier indexed Cargo Carrier T14 remained in paper. Permission was given in the summer of 1942 when the main vehicle was approved. The vehicle was similar to the GMC M12, but without a weapon and with a wider fighting (or rather cargo) compartment. It carried 40 rounds of ammunition. One of the main distinctive features was a circular rail with a .50 cal Browning M2HB machine gun. 100 vehicles were ordered, one per GMC M21.

Cargo Carrier M30 after modernization. This is how it went to the front lines.

The Cargo Carrier T14 was standardized much later, towards 1944, when the time came to modernize this vehicle as well. It was indexed Cargo Carrier M30. Like the GMC M12, Baldwin Locomotive Works was responsible for the modernization. 74 carriers were modernized in all.

Indirect or direct fire

As mentioned above, Field Artillery Command treated this new arrival with scepticism. New SPGs were issued to training units in order to figure out how they measured up to requirements. As a result, none of them saw battle in North Africa, Sicily, or Italy, unlike the GMC M10 that entered production around the same time. Perhaps if even a battery of M12s was sent to the front then Field Artillery Command would change its mind, but history doesn't know the word "if". The resulting situation was comical. The Germans, who put their heavy SPG into production later than the Americans, began using the Hummel in battle a year earlier. A year is a very long time in war.

The first SPGs of this type were issued towards the end of 1942, but spent more than a year and a half in reserve.

The Americans did a better job with their requirements. The placement of some crewmen outside of the vehicle and small ammunition capacity had its drawbacks, but the GMC M12 had many advantages as well. The vehicle weighed 27 tons, less than the tank whose chassis it was using. This meant that mobility was high. Formally the top speed was 40 kph, but this limit was set to protect the running gear. Trials of the M4A2 in the USSR showed that it was possible to accelerate to 50 kph, and the American SPG crews had the same ability. The Hummel could not boast the same. The chassis ended up overloaded, plus it was considerably taller. The GMC M12 also had a spade, which made it a more stable firing platform.

These vehicles only made it to the front lines in June of 1944, quickly earning the reputation of a potent mobile artillery platform.

The opinion of Field Artillery Command began to change in early 1944. The cause of this was the Siegfried Line, a line of fortifications that required mobile and powerful SPGs to assault. The GMC M12 made its debut in Normandy with the 557th and 558th battalions. Each had 2 batteries of 6 SPGs apiece. In total, the GMC M12 was issued to 6 artillery battalions, some of which used to have towed guns (258th, 987th, 989th, 991st). They arrived in Normandy later.

Some SPGs had supports for a tarp, but not all.

The effectiveness of self propelled artillery revealed itself in the first battles. Thanks to their high mobility, the GMC M12 could be quickly moved to new positions and support advancing troops. This proved useful during the assault on Cherbourg and during Operation Cobra. Towed guns could only dream of such mobility. SPG battalions were attached to armoured divisions, so the Americans and Germans had similar ideas about heavy SPGs. The GMC M12 also had a much greater range than the HMC M7, the most common American SPG.

Direct fire at the Siegfried Line.

 In war, vehicles are often used in unexpected ways. This applied to the GMC M12 as well. In the fall of 1944 these were among the first guns to fire at targets on German territory. The M12s did not fire at the Siegfried Line over the horizon. These SPGs were positioned for direct fire, and 1-2 hits was often enough to put a whole pillbox out of commission. This did not happen every time, and in some cases dozens of shots were needed. The use of the GMC M12 for destruction of heavy fortifications was a common mission.

The GMC M12 made a big impact in the history of American SPGs despite their low numbers.

The career of the GMC M12 ended almost immediately after Germany's surrender. At this point the US had new SPGs with the more powerful M1 gun. The M12 was quickly written off. Only one vehicle of this type survives to this day. One can confidently say that in this case, the first time was indeed the charm. The first mass produced American heavy SPG proved itself, including during tasks that it was not designed for. One can only wonder at the stubbornness of American field artillery commanders that delayed the debut of the GMC M12 for so long.


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