M40 GMC – 155 mm on a Sherman

The M40 Gun Motor Carriage (GMC) was a self-propelled gun that was designed towards the end of the Second World War as a replacement for the older M12 GMC. The M40 had a powerful 155mm gun, but there would also be parallel development of its “big brother” in the form of the M43 Howitzer Motor Carriage (HMC) that was built on the same chassis but possessed a larger 203.2 mm howitzer.

Due to its late arrival, the M40/M43 would only see very limited action towards the end of that war, but both would see extensive action during the Korean War. Both vehicles are also unique in that they were some of the last US Army self-propelled guns (SPGs) that were designed with an open roof.


Background and Development

The M12 GMC was introduced into the US Army in 1943 as one of the first 155 mm equipped SPGs and was built on the M3 Grant tank chassis, possessing a gun designated the M1917/1918.

This arrangement meant that the gun could be more easily transported and could be readied to fire even quicker than a towed version of the same gun. It also meant that the gun’s mobility was significantly improved, and was better able to move forward to support advances on the front lines.

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However, at their basis their guns were French Canon de 155mm Grande Puissance Filloux, a World War I field gun that was starting to show its age during the early 1940s. Also, at this stage of the war, the M3 was being replaced more and more with the M4 Sherman. Interestingly, the US military did not consider such a system as critical at the time, so only 100 were built.

T6, prototype for the M12 GMC.
The M12 prototype, the T6. Not the M3-style suspension bogies with the centrally mounted return rollers.

Once in service more were requested, but this was not possible due to low stocks of M3 hulls and the M1917/M1918 guns.

Therefore, an entirely new SPG would be required and the US Army began to study other howitzer and chassis combinations in 1943. An obvious choice for a chassis would be the ubiquitous M4 Sherman. Later components, like the HVSS system (Horizontal Volute Spring Suspension) and its corresponding wider 23 inch tracks would be selected from the outset.

In terms of an armament, a new 155 mm towed gun had been introduced into service during the North African campaign, the M1 “Long Tom”, so it was decided to mate the M1 with the M4 (US designation standards doing their magic here) to develop a new SPH. The Army had previously considered mounting the M1 on an M3 chassis, but the gun was too heavy with too much recoil.

155 M1 Long Tom.
The 155 mm M1, better known as the “Long Tom”, in its field gun arrangement.

The first two pilot models were designated the T83 and sent to Aberdeen Proving Grounds for testing in August 1944. At the same time, two other pilot models using the same chassis were equipped with a 203.2 mm (8-inch) howitzer (also designated the M1) and designated the T89 HMC. A third pilot model type was designed, designated the T30.

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This was a cargo carrier developed to carry extra ammunition and crew. Tests were successfully concluded in January 1945, with the T83 re-designated the M40 and the T89 as the M43. Another M40 variant, the T94 Mortar Motor Carriage (MMC) equipped with a 250 mm mortar, was designed in February 1945 and tested in 1946 but was never adopted.

M40/M43 Design

The M40/M43 were both based on the M4A3E8 HVSS Sherman chassis, however both vehicles required having their respective chassis widen to accommodate the 155 mm M1 (either the M1A1 or M2 variant) or the 203.2 mm M1 howitzer. Also, the engine would be moved from the rear of the chassis as it would be on a Sherman tank and moved closer to the front. The Ford GAA gasoline V8 engine that equipped the M4A3 variants would be replaced with a 9-cylinder Continental R975 air-cooled radial engine that produced 400-450 hp.

A spade was fitted to the rear of the M40/M43 to absorb the recoil from firing the gun. A working platform under the breech was also installed.

The HVSS suspension comprised three pairs of bogies that provided each bogie with two double-road wheels, a pair of back idlers, a pair of front sprocket-wheels and five pairs of return rollers. Tracks assembled with HVSS were broader than those assembled on vertical suspension (23 inch versus 16 inch on VVSS suspension) and HVSS provided a vehicle with better stability and ground pressure. As well, the horizontal suspension was more capable of absorbing the recoil of larger guns like the 155 mm and 203.2 mm.

M40 from the side.
The M40, with the 155 mm M1 gun. Note the spade at the rear, which helps transmit recoil from the gun into the ground. Image by 270862 CC BY-ND 2.0.

Both the M40 and M43 possessed an open top design, similar to American tank destroyers, but in theory this should not have been an issue for the SPGs as they were designed to provide indirect fires from long ranges, where tank destroyers would often be engaged in close combat where the open turret design was vulnerable to small arms fire, shell splinters and grenades.

Although combat experiences with the M12 would reinforce the need for open-topped SPHs to have armoured covers, and some covers were trialled for the M40, no tops were ever adopted.

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The 155 mm M1 gun had a range of approximately 14 miles (23 km) and fired a 43 kg projectile with high-explosive, armour piercing or white phosphorus ammunition. The 203.2 mm M1/M2 howitzer possessed a similar range but only fired high-explosive rounds. As a side note, a tactical nuclear round would be developed for the towed M115 in the 1950s.

M43 HMC, sibling to the M40.
The Howitzer Motor Carriage M43, armed with the 203.2 mm howitzer. Note the shorter but wider gun, this is the most obvious indicator that this is an M43. Image by Vincent Jackson CC BY-SA 4.0.

The projectiles for both guns were two-parts: the projectile itself and separate charge bags. Because of difference in shell sizes, the M40 could carry 20 155 mm rounds while the M43 could carry 16 203 mm projectiles.

Neither the M40 nor M43 had a secondary machine-gun for local defence. Instead, the T30 cargo carrier designed to accompany both SPHs had a gun ring for a machine gun, to be used by the assistant driver. In addition, both vehicles were equipped with 10 anti-tank grenades, and 12 hand grenades. The T30 could carry half of the M40/M43’s eight crew members while the remaining four would ride in the SPH itself. The T30 could also carry 100 155 mm or 66 203.2 mm rounds.


As the M40 and M43 were accepted into service in early 1945, neither vehicle would see much action during the Second World War. One M40 and one M43 would be assigned to the 991st Field Artillery Battalion for the Battle of Cologne in early March, 1945. Of note is that the assigned M43 would have its 203.2 mm howitzer switched out for the 155 mm M1 used by the M40 for unspecified reasons (this M43 would have its gun converted back to the 203.2 mm after the war).

While the M40 and M43 were considered to have performed well during the Battle of Cologne, the war in Europe would end in early May and production of both vehicles would cease shortly thereafter with 311 M40 (out of an initial order of 576) and 48 M43s having been completed. Eventually, 24 M40s would be converted to the M43, replacing the 155mm M1 gun with the 203.2 mm M1.

Only 5-6 T30 variants were ever produced as the US Army decided that existing cargo carriers and tractors would suffice for hauling extra ammunition and crew.

T30 cargo carrier, which would accompany M40 and M43s.
The T30 cargo carrier, based on the M40.

Although the M40/M43 arrived too late to see any prolonged action in the Second World War, both vehicles would be used extensively during the Korean War, mainly assigned to fixed emplacements along the 38th parallel. The open topped design would actually be advantageous in Korea as both vehicles were able to elevate to a high angle of fire, enabling their guns to hit targets on the rear slopes of the many steep hills in the country.

Despite their success in Korea, the M40 and M43 would be retired from American service shortly after the Korean War ended. In part, this was due to prevalent military attitudes of the 1950s that tactical nuclear weapons would likely be used in any conflict between NATO and the Warsaw Pact. Therefore, the open top design of the M40/M43 would be ill-suited for a radioactive battlefield and while some open top SPHs were developed afterwards by NATO, the majority of newly designed SPHs would be fully enclosed to increase surviviblity.

M40 GMCs firing in the snow.
A pair of 155 mm M40s open fire in Korea.

The M40 would still be donated to some countries after the Second World War under the Military Assistance Program (MAP), with the UK receiving some M40s, where it was designated the 155mm SP, M40 and nicknamed the Cardinal, a British Army tradition of using Church-related names for some of its self-propelled artillery such as Deacon, Priest, Bishop and Sexton.

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Several M40s would also be provided to France and some of these would be used during the First Indochina War until the French were defeated in 1954. There is no documentation of further use of the M40 (or the M43) after the First Indochina War, so it appears that it was quietly retired from service during the mid-late 1950s.


Length: 7,100 mm (23 ft 4 in) (M40, M43 – excluding gun)
Width: 3,150 mm (10 ft 4 in) (M40, M43)
Height: 3,300 mm (10 ft 10 in) (M40, M43)
Crew: 8 (M40, M43)
Armament: 155 mm M1A1 or M2 (M40), 8 inch Howitzer M1 or M2 (M43)
Weight: 36,300 kg, 36 tonnes (80,000 lbs, 40 tons)
Powerplant: Continental R975 C4 radial petrol engine, 16 litre, 450 hp
Top Speed: 24 mph (34 kph)