Gun Motor Carriage M10

GMC M10 at the NIIBT proving grounds, September of 1943. The photo shows mounts for applique armour, which was never installed.

By that point, the existence of the GMC M10 was no secret. Major Barayev, an assistant of the military attache in the US, reported about them on March 1st, 1943, but Khryaev's report arrived at GBTU faster. According to Barayev's report, the vehicle was developed to be universal, with the potential to complete objectives usually meant for tanks. For this purpose, it had mounts where additional armour could be attached. This was correct, but in reality, the armour was never attached to production vehicles.

The Americans also shared information about the use of the GMC M10 in combat with the attache's assistant. According to the report, the first 10 tank destroyers were sent to North Aftica in late 1942. Its use in battle showed that the design was good. It was assumed that it would not only be used by tank destroyer units, but by all armoured units in the US Army.

The same vehicle from the front.

In reality, the real combat debut of the GMC M10 was a bit later. These vehicles played a decisive role during the battle at El Guettar on March 23rd, 1943, and proved their superiority to GMC M3 halftrack tank destroyers. The Americans were honest in their assessment of the design. The fact that the GMC M10 was the most numerous American tank destroyer speaks volumes.

The same vehicle from the back.

The GBTU was not particularly interested in these vehicles, even though Barayev wrote that the firepower of the 76 mm gun in the GMC M10 was equal to that of the German 88 mm AA gun. That line in the report was underlined. This is not a coincidence, as the first trials of a captured German Tiger were held in the spring of 1943, and they showed rather alarming results. Soviet tank artillery was almost helpless against it. Means of fighting Tigers were needed urgently.

Another point in the American tank destroyer's favour was that it was built on the same chassis as the Medium Tank M4A2, which began arriving in the USSR since late 1942.

The tarp that was used on the march can be seen in this photo.

The issue of ordering these tank destroyers returned in early June of 1943. According to a report from the People's Commissar of External Trade, A.I. Mikoyan, addressed to Stalin, there were different opinions about buying SPGs. Yakovlev, the head of the GAU, considered buying the GMC T48 to be a priority, while all other vehicles should be acquired only for familiarization. Fedorenko, the head of the GBTU, wanted to purchase a wide spectrum of self propelled artillery, but the GMC M10 was not on his list. Voronot, the commander of artillery, had a separate opinion. He proposed the purchase of 50 "three inch M-10 guns on the chassis of a medium tank". The result was a compromise. A decision was made to order 2 tank destroyers, and a batch of 50 would be ordered depending on the results.

The delivery of the tank destroyers was prompt. On August 3rd, 1943, Khryaev reported that a convoy arrived in the Persian Gulf carrying two "M-10 tank-destroyers". Soon, the vehicles made it to Baku. There their paths diverged. One was sent to the NIIBT proving grounds at Kubinka, the other to the Gorohovets ANIOP. This was logical: NIIBT would test the mobility of the vehicle and ANIOP would test its armament.

However, there was an issue, since no American 76 mm shells were sent to the USSR. As a result, the gunnery trials happened a lot later than the mobility trials.

The stripes visible on the hull are the remains of the "packaging" that the vehicle was wrapped in during its passage across the ocean.

The American tank destroyer arrived at Kubinka in September of 1943. As with the ANIOP, the vehicle arrived without ammunition. The NIIBT proving grounds also reported that the vehicle was missing additional armour, but, as you remember, the Americans didn't have it either.

To start, the proving ground specialists studied the vehicle and composed a detailed technical description. At the same time, the vehicle was serviced. This was not hard, as the vehicle was very similar to the M4A2 medium tank, which the testers were already familiar with. The American oil was replaced with its domestic equivalent.

The fighting compartment. The testers mistakenly wrote down that the commander sits to the left of the gun. In reality, his station was to the right.

The technical description was mostly dedicated to the overall layout of the vehicle. The chassis of the M10 was described very briefly, as it was almost identical to that of the M4A2. The driver's compartment and fighting compartment were also described briefly. The NIIBT staff considered the commander's station to be to the left of the gun, but this was actually the gunner's station. As with American tanks, the commander sat to the right of the gun.

The staff at Kubinka did not spent much time studying the fighting compartment, and focused on other issues. The conclusions made about the crew conditions were mostly positive. However, the open topped turret was not met with enthusiasm, as reports of SU-76M crews being killed were arriving from the front. The NIIBT specialists also studied the visibility of the M10, which was deemed good.

Browning M2HB AA machinegun mount.

The main part of the program was not the study of the design, but mobility trials. According to the plans, the tank destroyer would travel 1000 km, 300 on a highway, 500 on dirt roads, and 200 off-road. Aside from the maximum and average speed, as well as fuel consumption, the mobility on various types of terrain would be investigated.

Visibility and dead zone diagram.

The mobility trials took place from September 24th to October 14th, 1943. Reality almost matched up with projections: out of 1000 km, 302 were spent on the Moscow-Minsk highway, the rest on dirt roads and hills. Trials on the highway coincided with measurements of the top speed. The top speed achieved across four tries was 50 kph, which is a little higher than official data. The average movement speed was 37.5 kph, and the average overall speed was 30 kph. The tank destroyer mostly drove in 5th gear. Fuel expenditure was 158 L per 100 km. To compare, the M4A2 consumed 162 L per 100 km. The driver was well positioned and it was easy to steer the vehicle.

Without spurs, the biggest slope that could be climbed was 22 degrees.

The M10 traveled 660 km on dirt roads, and most of it was done in 4th gear. 32 hours were needed to cross this distance, of which 8 were spent waiting for technical reasons. This was caused by destruction of road wheels, 6 of which were lost during trials. In five cases, the ball bearings were destroyed, in the sixth case, the rubber rim was destroyed. The ball bearings broke because of defective grease nipples, which allowed the grease to leak out and dirt to get into the ball bearings.

Despite the breakdowns, the tank destroyer behaved well on muddy dirt roads. The average movement speed was 20.5 kph, the average overall speed was 16.5 kph. Fuel expenditure was 259 L for 100 km, which was a little higher than on the M4A2 (246 L per 100 km).

Maneuverability was also tested. The tank destroyer was agile, however, it performed poorly during sharp turns, 180 degree turns, and turning at low speeds. This was not a problem unique to the M10, but a characteristic of all tracked vehicles with a double differential turning mechanism.

The final part of the trials program was crossing of hills. Here is where the problems started. The M10 arrived with rubber-metallic T51 tracks. These tracks, as well as the T41 used on the Medium Tank M3, had issues with off-road performance. The smooth track links could not grab the ground. This was critical when it came to climbing hills. The steepest hill that could be climbed was 22 degrees. After that, the tracks did not have enough traction.

Climbing a hill with spurs installed.

American engineers knew about this problem, so the tank destroyer came with special spurs. 11 spurs were installed on each track. This helped, but only partially. The tank destroyer could climb a 24 degree hill and a 28 degree hill. However, when climbing a 31 degree hill, the tank destroyer began helplessly digging at the dirt. There was not enough traction yet again. The maximum tilt was established at 22 degrees, after which the tank destroyed started to slide down. This was done without spurs.

The overall verdict on mobility trials was positive.

M10 tank destroyer at the Gorohovets Proving Grounds, March 1944

Firing was done with M62 armour piercing tracer ammunition. Conclusions were made that were not kind to equal caliber domestic ammunition. It turned out that the American shells were good enough to penetrate 120 mm of armour, while Soviet ammunition of the same caliber could only penetrate 100 mm. This was explained by the superior design of the M62 shell, better choice of materials, and better thermal treatment. The M62 shell could penetrate 120 mm of armour at 500 meters. Precision trials also gave satisfactory results.

Overall view of the M7 gun used in the GMC M10.

Of course, a tank destroyer does not just consist of the gun. ANIOP specialists inspected other parts of the vehicle, as this is where they found issues. Strangely enough, the artillerymen had complaints about the driver's station. According to them, the presence of only one periscope decreased visibility. To be fair, the Medium Tank M4 initially had two periscopes, but combat showed that periscopes in the upper front plate should be removed.

M62 shells after being fired at a 100 mm plate.

The evaluation of the fighting compartment was much more complete. A separate five page report titled "Notes on the issue of servicing the American 3-inch M7 self propelled gun". The first complaint was the absence of a roof. Here, tankers and artillerymen had the same opinions. To be fair, some American M10 crews installed their own roofs on their tank destroyers in 1944-45.

However, that is not the only thing Soviet specialists found deficient. Since there were no hatches in the turret, entering and exiting the fighting compartment was unsatisfactory from the point of view of safety. ANIOP specialists predicted crew losses when leaving the tank destroyer under enemy fire.

This was not the end. Unlike the NIIBT proving grounds, the ANIOP staff placed their crewmen correctly. Their positions were deemed inadequate. For example, the commander had a vertical aiming mechanism flywheel, electric trigger, and a panoramic sight, but he could not aim the weapon horizontally. The gunner had a full set of aiming flywheels, but they were positioned poorly, as they were too close to each other. Even though they were easy to turn, it was difficult to turn both at once. The telescopic sight was placed in such a way that the gunner's chest was right up against the flywheels. It was impossible to aim the gun and look through the sight at the same time.

A diagram of the fighting compartment layout included in the report.

There were also complaints about the loader's station. His position to the right of the gun was fairly deemed poor. There was a proposal that he should be placed to the left. The foldable seats were also found unsatisfactory, as they were hard, uncomfortable, and placed too high up (68 cm). Loading the gun was also inconvenient. The first six shots were in the ready rack in the rear of the turret, the rest were housed in cases in the sponsons. When the ready rack was used, the rate of fire was 10-12 RPM. As soon as those rounds ran out, the rate of fire dropped to 4-6 RPM. 

The results of the trials were mixed. On one hand, the conclusion was that the "American M-7 self propelled gun" passed the trials. On the other hand, the sizeable amount of complaints about the fighting compartment was not easily ignored.

It was clear that the GMC M10 would be used by the Red Army by late 1943, after the NIIBT trials were concluded. As agreed in June of 1943, the first batch was limited to 50 units. The first vehicles were expected to reach training units in early February of 1944. These were not the only American tank destroyers that began arriving in early 1944. A torrent of GMC T48 came, and the M10 was lost in it. 257 GMC T48s arrived in the USSR from December 14th, 1943, to March 5th, 1944. They received the name SU-57.

Late production GMC M10, NIIBT Proving Grounds, summer of 1944.

As with the first M10s, the new tank destroyers were shipped by the southern route, through Baku. The first vehicles arrived in January, but there were delays when sending them to the training center. Batches of 15, 22, and 5 vehicles arrived at the SPG Training Center in Klyazma (Moscow oblast) in the second half of February. According to photographs, the USSR received mid and late production tank destroyers, which had slightly different turret shapes. One late production vehicle later ended up at the NIIBT Proving Grounds.

A storm of correspondence brewed up regarding a very serious issue. The problem was that the tank destroyers were 80% equipped. Four M10s had no parts and toolkits at all, along with machineguns or optics. There were no company level parts or toolkits at all. Tank destroyer number 40110927 had a smashed clutch case, bell housing, and clutch disk. One other tank destroyer could not drive at all.

Sergeant A.M. Ryazanovskiy from the 37th Independent Tank Regiment with an M10 in the background, 1944. It's possible that this is one of the six vehicles in the last batch. The M10 already has T54E1 track links with improved grousers.

The management of the SPG Training Center sounded the alarm. Soon, a letter signed by the chief of the facility, Major-General of Artillery, N.S. Kasatik was sent to the GBTU about a different issue. The chief reported that the crews that are arriving are very poorly prepared. Some of them saw the tank destroyers for the first time. For example, he 15 drivers and gunners that arrived on March 4th did not know the vehicle at all. The same thing happened with 25 more crews that arrived on March 9th and only learned the SU-76M before that.

There is nothing surprising about that, as no literature was prepared on these vehicles. The chief demanded that officers familiar with the vehicles should be sent immediately to resolve this issue and that the center should be urgently equipped with manuals. The demand for trained officers was a bit much, since the only people familiar with the M10 were specialists from the proving grounds.

The situation slowly improved, but organizational problems delayed the arrival of M10s on the battlefield. It's worth mentioning that only 875 AP shells were available on January 27th, 1944, and no HE shells at all. 5000 shells only arrived in early March. Additional supplies of ammunition were set up a little later. The GBTU USA approved an ammunition loadout for the M10s on April 26th: 27 armour piercing and high explosive shells each per vehicle.

There are very few images of the "American SU-76". In this case, it's a vehicle from the 1223rd Self Propelled Artillery Regiment.

The last 6 vehicles arrived in Baku in late February of 1944, and were sent to Pushkino on March 8th by train #33387. On June 1st, 1944, these tank destroyers were still listed as in reserve. As for the other 44 M10s, they were sent to the 1239th and 1223rd Self Propelled Artillery Regiments. These vehicles were given the index SU-76, which was already taken by the Soviet SPG, and makes studying their experience in combat difficult.

It's known that the 1223rd regiment participated in the liberation of Belarus and the Baltic as a part of the 29th Tank Corps, 5th Guards Tank Army. All additional information is contradictory. According to some data, the M10s were already gone from the regiment by the time they reached the Baltic, other information states that 10 vehicles were still in the regiment on May 1st, 1945, 4 of them functional. Another document, dated February 7th, 1945, indicates that 37 SU-76es were present in the regiment, 18 of them functional. It's hard to say whether these are Soviet SU-76es or their American namesakes. As for the 1239th regiment, it actively participated in the liberation of Poland as a part of the 16th Tank Corps, 2nd Tank Army.

There were no further requests for M10 tank destroyers. It was known that the M4A2(76)W medium tank entered production in the summer of 1944, and the first tanks began arriving in the USSR in September. Carrying practically the same gun, they were superior to an open-topped SPG.