Monday, 8 November 2021

Mobile American Howitzer

The American army had a relatively small amount of armoured vehicles on hand at the start of WWII. Just over two years of neutrality proved enough to fully prepare for war. The American army had an impressive amount of light and medium tanks by December of 1941. They had so many tanks that they were also able to supply the British and offer significant help to the USSR. By the time the United States entered WWII they also had their first SPGs. At first these were built on halftrack chassis, but half a year later fully tracked vehicles became available. One of them was the HMC M7, better known under the British designation Priest. This was the most common self propelled howitzer of the war and it remained in service for decades after.

Experiments on a medium chassis

The Americans first worked on SPGs during the First World War. That work did not progress past experiments and work stalled after the war ended. An attempt was made to build an SPG on the chassis of the Light Tank T1, but only one prototype was built. Further work focused on vehicles equipped with the 75 mm Pack Howitzer designed for direct fire roles. 

It became obvious towards the summer of 1941 that another self propelled howitzer on a larger chassis was needed. The light chassis could not mount the 105 mm M2A1 howitzer. This system had a long history of development. The first models appeared in the late 1920s, but mass production only began in 1941. The wait was worth it, as the M2A1 was one of the best guns in its class. Suffice it to say, this weapon is still in service. The idea of mechanization was an obvious next step. This gave the weapon better mobility and significantly decreased deployment time. These guns could quickl change positions to support advancing forces.

The M2A1 howitzer was one of the main types of American artillery during the war.

The first attempt to mechanize the M2A1 took place in June of 1941. Requirements for the Howitzer Motor Carriage T9 called for using the chassis of the Medium Tank M3 that was just put into production. The speed, armour, and cruising range were to be the same as the M3. The height was limited to 2032 mm. The maximum gun elevation was to be 20 degrees and gun depression at -10 degrees. These kinds of parameters suggest that the weapon would be used for direct fire missions. Unfortunately, nearly no data remains on the HMC T9, but it was generally similar to the GMC T24 that appeared later. The project was abandoned quickly, as the HMC T9 had little promise. Instead, the HMC T19 on the M3 halftrack chassis was developed very quickly. Mass production of the HMC T19 began in January of 1942 with 324 vehicles built in total.

HMC T32 on trials, February 1942.

The HMC T19 was a necessary compromise. The Ordnance Department understood perfectly that this vehicle was just a bandaid solution. For instance, it carried only 8 rounds of ammunition on board, plus the M3 chassis was too weak for such a powerful gun. Head of the Armored Forces Major General Jacob Devers authorized the development of a new SPG on the Medium Tank M3 chassis in October of 1941. This vehicle was indexed HMC T32. It was designed with mistakes of the HMC T9 and GMC T24 in mind.

The HMC T32 was not without fault, but it still proved preferable to the HMC T19 halftrack.

Baldwin Locomotive built two prototypes that were conceptually different from the T9/T24. Since the requirements called for a low silhouette, the gun was sunk into the casemate as far as possible. The vehicle was still very tall, 2440 mm. However, the maximum gun elevation increased to 35 degrees. The traverse range to the right increased to 23 degrees since the gun was shifted to the right slightly. The ammunition capacity was quite decent: 44 rounds. The HMC T32 chassis was also used to design the GMC T40 tank destroyer which was even briefly standardized as the GMC M9.

The second HMC T32 prototype modified to account for experience gained in trials.

The first prototype was sent to Fort Knox in February of 1942, shortly after it was tested at the Aberdeen Proving Grounds. A large list of changes was composed as a result of these trials. The tankers' desires sometimes conflicted with reality, for instance they wanted the M2A1's full elevation (66 degrees) and a low silhouette at the same time. This was physically impossible. All that could be done was to increase the traverse to the right to 30 degrees. The silhouette of the HMC T32 changed dramatically. The initial vehicle had a casemate similar to the hull of the Medium Tank M3, but in the final configuration it was greatly enlarged. The fighting compartment became larger and could now fit not just the full 7 man crew but also 57 rounds of ammunition.

The ammunition was also rearranged. Previously all rounds were stored in one large rack, now most of it was kept in bins under the fighting compartment floor. The thickness of the casemate armour was reduced to 12.7 mm, but the vehicle's mission didn't require any more. Another significant change was the introduction of the M2HB .50 cal machine gun in the front right corner of the fighting compartment. All of these changes were introduced into the second HMC T32 prototype that remained at the Aberdeen Proving Grounds. Despite all drawbacks, it was clear that the HMC T32 is greatly superior to the HMC T19. For this reason, the decision to put this vehicle into production was made by March of 1942. By April it was standardized as the HMC M7.

A tricky platform

The need to put the HMC T32 into production as fast as possible was understandable. Unlike tanks, the Americans were having a tough time with SPGs. The light self propelled howitzer program failed completely and the vehicles it produced had quite questionable characteristics. Halftrack chassis had their own problems: low ammunition capacity, tall silhouette, cramped fighting compartment. The HMC T32 gave the Americans a fighting vehicle with good ammunition capacity and characteristics that allowed support with indirect fire.

The first production HMC M7, Aberdeen Proving Grounds, April 1942.

The American Locomotive Company (ALCo) was selected to produce these vehicles. This company had been building the Medium Tank M3A1 since January of 1942. The chassis of the tank and SPG were identical. ALCo's M3 tanks had a cast upper hull, but this was a temporary deviation. By the spring of 1942 it was clear that the M3A1's days are numbered and the upcoming Medium Tank M4 was going to have a fully welded hull, so ALCo was going to have to master welding anyway. The casemates of the first HMC M7 vehicles that arrived at the Aberdeen Proving Grounds in early April of 1942 were already welded. 

Unlike the experimental T32, the production M7 had a welded casemate, although the lower hull "tub" was still built with rivets.

Like the HMC T32, the M7 carried 57 rounds of ammunition. The casemate was built in a similar fashion as well, but significant changes were introduced from the start. The AA machine gun ring was now different and armoured protection was introduced along its perimeter. Racks for grousers were added to the front of the casemate and bins for personal belongings were added to the rear. The engine deck of the first M7s also changed.

Demonstration of the Browning M2HB machine gun.

The British Military Mission kept a close eye on the HMC M7 from the very start. The British had no equivalent vehicle and their SPG program was also looking quite sad. To make up for it, they ordered an impressive volume of these new vehicles: 2500 HMC M7 in 1942 and 3000 in 1943. Much like the requirements for M3 tanks these demands had little to do with reality, but in the end it was the British who used these weapons first. Later the British began using an analogous Canadian design, the Sexton.

A removable roof was tested on the fourth production vehicle. Take note of the engine deck, only the first 100 M7s built had an engine deck like this.

According to contract T-3529 ALCo built 599 vehicles with registration numbers U.S.A. W-3034235-3034833. Visibly noticeable changes were introduced from practically the very beginning of mass production. The engine deck changed after the first 100 vehicles. Air intakes protected by railings were added. These changes were introduced as a result of trials of the first two production vehicles, presumably trials showed that the engine overheated. The first list of changes was ready by May 5th, 1942, but improvements were introduced gradually. The engine deck was changed first, but that was far from the only request.

The first major change: an engine deck with additional air intakes.

Additional changes to the fighting compartment were also requested. 57 rounds of ammunition was not enough and a proposal was made to increase that number to 69. 7 additional rounds were stowed along the left side and 5 along the right. This made fitting seats complicated. The issue was solved simply by removing 4 of them. This change was not made right away, but sometime in the summer of 1942. The toolbox lids were also changed for an unknown reason, but early lids can still be seen on later vehicles.

A one piece transmission cover was introduced in the summer of 1942

The chassis of the HMC M7 changed gradually as well. As mentioned above, the chassis came from the Medium Tank M3, but it replacement in the form of the M4 was coming. The HMC M7 began to transform as a result. The transmission cover was the first sign of change. Initially it was made from three pieces bolted together. A single piece cast cover was introduced in the summer of 1942. This cover better resisted gunfire, but the old cover was still installed until the backlog was expended.

Ammunition racks changed, increasing the ammunition capacity to 69 rounds. A portion of the crew's seats were removed to make room.

The last vehicles to satisfy contract T-3529 were delivered in September of 1942, after which the second contract, T-3882, came into effect. ALCo was to deliver 2214 SPGs with registration numbers U.S.A. W- 4037519-4039732. Medium tanks M4 and M4A2 ordered by contract T-1480 also entered production in September. It was around this time that the HMC M7 received bogeys from the M4, even though the hull tub was still the riveted M3/M3A1 style. The M7 transitioned to more and more elements taken from the M4 rather than the M3. ALCo unified its chassis by early 1943. The three piece transmission cover made a comeback, although it was later replaced again. New headlight guards were added during production.

The process of unification with the Medium Tank M4 began in the fall of 1942. The M4's suspension was used and the three piece transmission cover returned.

The casemate was also undergoing an interesting transformation. One of the causes was experience in using the HMC M7 at the Battle of El Alamein, although there were others. Recall that the HMC M7's fighting compartment was completely open to the elements, and so the crew suffered from precipitation. Experiments on a removable tarp roof began in the spring of 1942. Two wire supports for the tarp were installed. If the tarp was removed, they could be carried on the side of the casemate. This change was put into production towards the end of 1942.

Guards for the headlights and a new AA machine gun turret were introduced around this time.

A bigger issue discovered during battle was that the ammunition racks were not protected from flanking fire. The rounds stuck out above the sides of the casemates, inviting enemy bullets. An order was given to urgently resolve this. Special folding plates were added to the sides of vehicles that would protect the ammunition. Additional plates were welded on to existing vehicles. Another issue was the poorly placed machine gun turret. By early 1943 it was made taller, making it easier to work in. All of these changes slowly increased the mass of the vehicle, but it was still less than its base chassis. The combat mass hit 23 tons by the start of 1943, so the power to weight ratio was still higher than for the Medium Tanks M3 or M4. 

A tarp was added in late 1942.

Contract T-3882 was satisfied in August 1943, and ALCo focused on Medium Tank M4 production. It seemed that the HMC T76 on the Light Tank T24 chassis would replace it. This vehicle was more mobile, had a more compact gun, but maintained the same ammunition capacity. Unfortunately, development dragged on, and these SPGs were needed urgently. Production of the HMC M7 had to resume. T-3882 was altered to call for 500 more vehicles with registration numbers U.S.A 4039733-4040232.

July 1943 production vehicle with late model changes like folding side panels. The suspension bogeys are the M3 type due to a backlog of parts.

Late production vehicles were definitely not based on the M3 chassis. They were built with all the changes made to M4 tanks, plus the lower hull tub was now welded. The last type transmission cover was also used. The casemate underwent many changes as well. Lights and the rack for grousers on the front plate changed. New bins for personal belongings were added. The gun was equipped with a new better travel lock. ALCo produced the HMC M7 in this form until October of 1944.

1944 production HMC M7. This vehicle was very different from earlier ones and the chassis here is all taken from the Medium Tank M4.

Augmentation of contract T-3882 wasn't the only action taken to renew HMC M7 production. There was a great need in these vehicles, not just for the American army but also for Lend Lease shipments. It was necessary to increase the number of manufacturers. The chassis of the Medium Tank M4 was no longer optimal at that point. The Medium Tank M4A3 with its more powerful and simpler Ford GAA engine was a better choice. The decision was made to put the HMC M7 into production on a new chassis.

500 HMC M7 SPGs were built in 1944. 176 more were built in 1945 by Federal Machine & Welder Company.

One of the original makers of the M4, Pressed Steel Car Co. Inc. was included in the list of M7 manufacturers. The company signed contract T-10154 for 628 vehicles with registration numbers U.S.A. 40152252-40152879. Pressed Steel began working on this contract in March of 1944 and finished in December. An additional 198 vehicles of this type (registration numbers U.S.A. 40172420-40172617), were built from December 1944 to February 1945 according to contract T-10813.

Production of the HMC M7B1 began in March of 1944. These vehicles were built using the chassis of the Medium Tank M4A3.

The vehicles produced by the Pressed Steel Car Co. had a different index. While ALCo kept calling its vehicles HMC M7 even when the chassis was completely different, the SPGs on the M4A3 chassis were named M7B1. There was no change in the fighting compartment, all the differences were in the rear. The mass turned out to be slightly less than the M7 (22,860 kg) and the top speed was limited to 40 kph. This was an artificial limit, technically the HMC M7B1 could accelerate to 50 kph.

Differences of the HMC M7B1 can be seen from above or from behind.

The last vehicles in this series were built by a third manufacturer. Instead of ALCo, the government signed a contract with Federal Machine & Welder Company, a manufacturer of the Medium Tank M4A2. Federal built 176 vehicles with registration numbers U.S.A. 40185472-40185647 from March to July 1945. The contract was then cancelled for a number of reasons. First of all, the war was coming to an end. Second, the long-awaited HMC M37 was around the corner. Alas, production lasted only for a few months with 150 vehicles built, a drop compared to 4315 vehicles of the HMC M7 family, the most numerous self propelled howitzer of WWII.

Successful long-liver

Many American tanks and SPGs made their debut with a foreign army. This was true, for example, of the Light Tank M3 (Stuart). The Americans first used them at the end of 1941, but the British did it in September. The same thing happened with the medium tanks. The British used the Lee and Grant in the first half of 1942, and the Sherman II (Medium Tank M4A1) went into battle with British crews in the fall of 1942. American SPGs were no exception. Lend Lease deliveries began in the summer of 1942, and the British had about 90 of them during the defensive phase of the fighting at El Alamein.

The Battle of El Alamein was the Priest's trial by fire.

Let us stop here and address a myth about the HMC M7. Allegedly, it was nicknamed Priest due to its machine gun turret which resembled a pulpit. This assertion migrates steadily from book to book, but has absolutely nothing to do with reality. Everything is far simpler. The British introduced a system of names instead of indexes to avoid confusion. For instance, the Infantry Tank Mk.IIA* became the Matilda III. The same process applied to American vehicles. The Light Tank M3 was called General Stuart I, shortened to just Stuart I in 1942. The names of all SPGs, including wheeled ones, had a religious theme. The proponents of this naming theory for some reason forget, for instance, about the Bishop and Deacon. The theory that the name was only applied after the Battle of El Alamein is also false, as it pops up in British documents in August of 1942. The vehicles themselves arrived at Suez in early September. 

The British received over 800 vehicles of this type in total.

The British received 828 (832 in some sources) vehicles of this type. The initial order was much greater, but the Americans needed these vehicles for themselves. The appearance of the Sexton with its 25-pounder gun also resulted in a decrease in orders. The actual order was rather small and only lasted for 1942-1943, with the bulk of the shipments (497 units) made in 1942. The first shipment was made with 25 vehicles. It was these SPGs that fought at El Alamein. All 25 fit in the S.169299-S.169311 range of WD numbers. The first large shipment of 200 vehicles received WD numbers in the S.214438-214637 range. 300 vehicles numbered T.215638-215937 followed, then 300 with T.230470-T.230769. Three more vehicles with WD numbers T.238464-238466 are also known. The first use of the Priest in support of the 1st Armoured Division at El Alamein thrilled the British. This successful debut played a part in expediting the Sexton program. Even though the British had many Sextons by the summer of 1944, early Priests were still widely used.

HMC M7 during the fighting in Sicily.

The urge to help for the British backfired on the Americans. Having given away more than a third of the first production batch, they had a shortage themselves. These vehicles were supposed to be used in Army Field Artillery Battalions (AFAB). Each battalion had 3 batteries of 6 vehicles. Armored divisions, the higher priority recipients of the HMC M7, were supposed to have 3 battalions each. In practice, by the start of Operation Torch some of the battalions were still equipped with the HMC T19. The resulting situation was quite comical: tank divisions had the obsolete HMC T19 while infantry divisions already had the HMC M7. The M7 became a truly common sight by early 1943 thanks to increased production and its success on the battlefield.

Some of the vehicles had additional armour on the sides to protect the ammunition.

The HMC M7 were judged highly as fire support vehicles in Tunisia, but not without their nuances. For instance, Soviet tanks are often criticized for their lack of radios caused by wartime shortages. In case of the M7, it was never supposed to have a radio at all and didn't have a radio operator in its crew. The British had to install their own Wireless Sets No.19. There are some photos of American M7s with radio antennas, but officially communication was up to the battery commander who had an M3 halftrack for this purpose. This is a somewhat surprising approach to communications.

The HMC M7 was very common by the fighting in Normandy.

American artillery in tank units was rapidly reorganized thanks to increased M7 production starting in late 1942. These were the priority formations for assignment of the AFABs. Out of 62 battalions armed with M7s fighting in Europe, 48 were attached to tank units. The German TO&E K.St.N.431b was actually quite similar to the American one. It also had 3 batteries of 6 vehicles, although they combined 12 of the weaker Wespe with 6 of the more powerful Hummel. Both approaches had their advantages and disadvantages, but the American vehicles was superior to the German one in many ways. The HMC M7 had more than twice the ammunition of the Wespe, and the rate of fire reached 8 RPM thanks to a roomier fighting compartment and larger crew. A well organized battery could deliver firepower that matched a Katyusha salvo. The HMC M7 was beloved for its ability to quickly saturate a target with shells. The M7 was also more mobile than German SPGs. It was also a lot more numerous.

HMC M7B1 from the 11th Armored Division, 1945. The trailers came with the M7 and contain additional ammunition.

Direct fire was also not too rare for these SPGs. Of course, it wasn't the original intention of the designers, but the principle of "anything with tracks and armour is a tank" is universal. Even with its thin armour, the HMC M7 had a powerful gun, and so it was used as an anti-tank weapon. The M67 HEAT round provided it with enough firepower to defeat most German tanks. Using the M7 for this purpose was still officially discouraged, and so the SPG had lower casualty rates than tanks.

The HMC M7 was not as common in the Pacific. Vehicles considered obsolete in Europe still excelled here, plus the region needed howitzers in direct fire roles. The tall silhouette was not very helpful. The number of M7s fielded here increased in 1945, but they still fought alongside the HMC M8 that had a weaker gun but was still better suited for the specifics of the region.

These vehicles were rare in the Pacific.

The end of WW2 did not mean that this vehicle's service was over. The HMC M7 was still a modern fire support vehicle despite its shortcomings. Production of the HMC M37 was also cut as a result of the end of the war, so no replacement was available outside of a few units. As a result, the HMC M7 remained the most common American self propelled howitzer until the mid-1950s. The M7B1 was preferred due to its superior chassis. The M7 fought in Korea, where one of its drawbacks was very obvious. The desire to reduce its height as much as possible made it harder to use in mountainous regions.

HMC M7B2, a special Korean variant.

This drawback had to be resolved quickly. This resulted in a modernized variant of the vehicle, the HMC M7B2. The gun mount was elevated, increasing the maximum gun elevation to 65 degrees. Since the gun mount got in the way of the machine gun turret, it was also enlarged. The M7B2 conversion was not common, only 127 vehicles were modified. In every case the initial vehicle was an M7B1. These vehicles were only needed in certain regions, and most units were happy with the M7/M7B1.

Increasing the height of the gun mount allowed the gun elevation to be increased to 65 degrees.

The American army began actively disposing of the HMC M7 starting in the mid-1950s. 179 of these SPGs were given to the French back in the days of WW2, and they used them very actively. Yugoslavia also used the Priests that were given to them by the British. 56 units were purchased after the war. The HMC M7 was used by more than a dozen nations, some of which had the chance to use them in battle. The IDF used them in 1967 (Israel purchased 100 vehicles in total). Italy received their first vehicles indexed Semovente DA 105/22 M7 in 1947. The Italian army, who used mostly American vehicles after the war, ended up with 129 SPGs of this type. Like many other new owners, the Italians equipped their SPGs with radios and also made other changes. One of them was the Semovente DA 105/34 with a more powerful gun. The M7 was also used to built special vehicles including command vehicles.

Semovente DA 105/34, an Italian modification of the HMC M7.

The last SPGs of this type were taken out of service in the 1980s, which explains the large amount of M7s that survive today. The secret to such a long life was that the vehicles remained useful for the task that they were built for, even after many decades.

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