Sunday 3 June 2018

GMC M12: King Kong on Tracks

One of the best weapons of French origin adopted by the US Army in WWI was the 155 mm GPF cannon, designed by Captain Filloux in 1917. The gun had a simple, easy to service, reliable, and effective recoil mechanism, a significant horizontal aiming arc, and a long range. However, the GPF was fairly heavy. It was no accident that it was one of the first candidates for an SPG.

In 1919, ten Gun Motor Carriage Mk.II SPGs were built with the M1917 gun (that was the index given to French GPF guns) installed on an unarmoured chassis of a Holt tractor. These vehicles did not fight, but improvements continued. The modernized Gun Motor Carriage Mk.IX was tested in 1925. However, work on SPGs stopped in the USA for the next 15 years. The conservative top brass in the artillery branch opposed any such work. They had different arguments for this, both economical (the SPGs were rather expensive) and tactical. Towed guns were smaller than SPGs and were easier to camouflage. In the case of a breakdown, the SPG became completely useless, while a towed gun just needed a new tractor.

Only the start of a new war in Europe and experience with the Blitzkrieg in Poland and France convinced the conservatives that tactical mobility was increasingly important. When a good medium tank was designed (the future M3) in the second half of 1940, its chassis was to be used for increasing the mobility of 155 mm guns. The project was indexed Gun Motor Carriage T6. In June of 1941 the Ordnance Department ordered a prototype of the T6 to be built at the Rock Island Arsenal. Construction of a prototype was completed in February of next year, after which the vehicle was sent for trials at the Aberdeen Proving Grounds.


The T6 SPG kept the M3 suspension and lower hull. The engine also remained the same, the Wright or Continental R975-C1. However, the layout of the vehicle changed. The engine had to be moved to the middle to fit the massive gun in the rear. The fuel tanks were also moved. The M3 had them in its rear sponsons, but the T6's fuel tanks were in the center.

Gun Motor Carriage T6.

The SPG was armed with the 155 mm M1918M1 gun, the name given to GPF guns produced in the USA, which were slightly different from the original (such as in the design of the breech). About half a dozen early SPGs (including the T6 prototype) received the older M1917A1 guns, French-produced GPFs modified to the M1918M1 standard. The gun could be aimed 14 degrees to the left or right, and from -5 to +30 degrees vertically. The aiming was done by hand. Sights, which included the M53 telescope for direct fire, M6 panoramic sight, and M1918A1 artillery clinometer) were positioned to the left of the gun.

The gun was installed in the rear of the vehicle in an open fighting compartment. The size of the gun did not allow for sufficient space inside the vehicle, so the crew had to work from outside. The rear of the fighting compartment doubled as a hydraulically operated deployable trail. The SPG also came with clogs. The trail and clogs stabilized the SPG during firing and absorbed a part of the recoil.

The trail (or rather, its hydraulic drive) was the weakest link of the T6. After several shots the hydraulic cylinders deformed. The SPG became immobilized, as it was no longer possible to lift the trail. The design of the hydraulic drive and the trail had to be changed immediately. After the changes were made, the SPG successfully went through trials at Fort Bragg. The SPG demonstrated complete superiority over towed SPGs in terms of march speed, maneuverability, tactical mobility, and time to fire after reaching firing positions.

The ammunition consisted of separate propellant rounds with three types of shells: HE M101 weighing 42.96 kg, APCBC M112B1 weighing 45.36 kg, and WP M104 weighing 44.53 kg. The maximum range of the HE shell was 18,400 m. The penetration of the APCBC shell at 60 degrees impact was 127 mm from 500 yards and 119 mm from 1000 yards.

Four of the crew remained in the fighting compartment during the march. Two of them were located near the left side of the fighting compartment, sitting fairly high up. During the modernization, these positions received additional protection in the form of 19 mm armour plates. Two more crewmen sat on the sides of the gun breech. All positions were equipped with seatbelts.

The M12 was not equipped with a radio station. It only had a telephone to communicate when deployed and signal flags. There was also no internal comms, which made it difficult to communicate with the driver and his assistant, who sat in the driver's compartment.

Production and modernization

The SPG was recommended for production in July of 1942 and standardized as the 155 mm Gun Motor Carriage M12. At the same time, the Field Artillery Commission ordered an auxiliary vehicle that was supposed to carry ammunition and the rest of the crew. The first order for 50 vehicles was given to the Pressed Steel Car company even before the M12 was officially standardized. Another 50 SPGs were ordered in August of 1942. The first vehicles were ready by November. Production of the second batch was complete in March of 1943.

The first production M12 was used for additional trials. Several more were used for crew training. The rest were sent from the factory straight to the warehouse. Command expressed their desire to finish training the personnel before issuing the SPGs. This position might seem a little strange, since there was nothing new: the gun was used since WWI and the M3 chassis was well known by the troops. However, after learning of the successes of high caliber SPGs used by the Red Army and the Wehrmacht in late 1943, the Americans suddenly remembered the M12s sitting around in warehouses. It was decided to use them in the landings in Normandy.

It turned out that the vehicles became obsolete. The chassis was based on the Medium Tank M3, but the army moved to the Medium Tank M4, the famous Sherman, since then. If the M12 was used in Europe, then issues might crop up with supplies. It was decided to modify the M12 to equip it with as many M4 components as possible, while at the same time getting rid of the drawbacks discovered during training.

In December of 1943, the Americans decided to modernized 75 M12s to equip six artillery battalions(three four-gun batteries apiece with a three gun reserve). The modernization was done from February to May of 1944 at the Baldwin Locomotive Works. For some unknown reason, only 74 SPGs were modernized, not 75. The improvements included:
  • Replacement of the bogeys with M4 bogeys with reinforced springs.
  • Installation of a new trail with a hand-cranked winch. This design was less advanced than the hydraulic drive, and required significant effort, but it was more reliable and allowed for the trail to be raised higher to clear obstacles. The space between the trail carriers was also covered with metal plates that formed steps when deployed, which made the crew's work easier.
  • The batteries were moved to the front of the right sponson, which was lengthened. The assistant driver's hatch was removed.
  • 19 mm armour plates were installed around the sights.
  • An ammunition rack for 10 rounds was introduced (earlier the M12 carried no ammunition onboard).
  • A tarp to cover the fighting compartment during driving, protecting the crew and gun from rain and dust. In practice, this tarp was used very rarely, as it turned into a gas chamber when installed according to instructions if the engine was faulty. Crews frequently fashioned safer improvised tents.
A modernized M12 SPG.

Ammunition carrier

One of the M12's weaknesses was the limited amount of ammunition carried onboard. Initially, the SPG did not carry any ammunition at all. A decision was made in March of 1942 to create an ammunition carrier with the same mobility as the SPG. The design of the SPG served as the foundation for the carrier, indexed T14, and the prototype T6 was converted. In September the prototype successfully passed trials at Erie, and in October the Pressed Steel Car Company began work on the first batch of 50 carriers. There is no precise information regarding the total number of carriers produced. There could be as many as 100 (one carrier per SPG), but no fewer than 81, since that was the number of carriers that was modernized in 1944 before the landing in Normandy. The vehicle was standardized as Cargo Carrier M30.

Cargo Carrier M30, viewed from the left.

View from the right.

The M30 was similar to the M12, with the exception of the fighting compartment, where 40 rounds of ammunition were carried in racks. The propellant was kept in the right sponson behind the fuel tank (5 charges), in two containers on the floor of the cargo compartment (5 charges apiece), and in the left sponson (21 charges). Fuses were carried in separate boxes. Overall, the carrier transported 2.5 tons of ammunition.

Instead of a trail, the cargo compartment was covered with a stepped ramp, which made loading easier. The crew had to raise and lower the ramp themselves. The ramp was held in position with two chains. When raised, it could serve as a seat for three crewmen. The third sat in the end of the left sponson facing right.

The M30 was armed with a .50 cal Browning M2HB machinegun installed in a ring mount. 1000 rounds of ammunition were carried.

Use in battle

M12 SPGs were used to equip six artillery battalions: 557th, 558th, 981st, 987th, 989th, and 991st. All of them fought in Western Europe, with the 557th and 558th starting the fight in the first few days of the Normandy operation. The other battalions arrived later. The battalions were assigned to corps and were used as reinforcements for divisional artillery in both offensive and defensive roles. Pre-offensive artillery barrages and counterbattery fire were included among their roles. When fighting in France reached the maneuver stage, M12 battalions began to play an important role in pursuing the retreating enemy. With an average speed of 30-40 kph, they could cover 200 km per day. The M12 could reach a speed of 56 kph on a good road. A record march for an M12 battalion was 320 km per day. Frequent and quick changing of firing positions was drilled thoroughly during training, as well as direct fire. Both of these skills came in handy in real fighting.

GMC M12 landing in Normandy. The SPGs are equipped with additional protection against water.

The M12 fought alongside the Cargo Carrier M30.

One of the most interesting documents regarding the use of units armed with M12 SPGs is an article by Lieutenant Lewis R. Soffer from the 991st Field Artillery Battalion, published in the Field Artillery Journal in January of 1945. In this article, Soffer describes three months of fighting, from the end of June to the fall of 1944. He splits the time period up into four portions, each of which were characterized by a different type of fighting. The first phase covers the fighting in Normandy, when the M12s were used in the same way as all artillery of the 7th Corps of the 1st Army (which the battalion was attached to). During this time, the main objectives of the battalion were participation in the general fire missions, artillery barrages in breakthrough targets (especially during the assault on Cherbourg on June 26th), suppression of artillery batteries, and firing on infantry according to direction by forward observers. Fighting was happening with low intensity at fairly short ranges, so M12 batteries rarely changed positions. This allowed them to settle in, engineering-wise. The positions had trenches for the SPGs and personnel.

The SPG's impressive size earned it the nickname "King Kong". A vehicle from the 587th Battalion is shown. Bayeux, France, June 10th, 1944.

While preparing for the breakthrough to Saint-Lô (Operation Cobra, July 25th, 1944), corps level artillery received the task to allow aircraft to fly. The 991st Battalion suppressed German AA gun batteries, which harassed Allied bombers.

After the breakthrough past German positions at Saint-Lô and reaching operational freedom, the second phase of the 991st's fighting career began, characterized by maneuver. In August, the Allies tried to complete the encirclement of the German forces around Falaise-Argentan. The 991st Battalion was attached to the 3rd Tank Divisions from the 7th Corps. Lieutenant Soffer wrote:
From the first it became apparent that the weapon was more useful in this role than it had been in its previous one in general support of a corps effort. The battalion was able to keep up with the armored division on all displacements and was able to register within a reasonable time after each displacement.As a result the fires of the battalion were available to the DivArty commander throughout most of the action. At various times during the action other medium and heavy battalions were attached to the division, but the self-propelled battalion found it somewhat easier to displace and keep up with the division. 
An M12 SPG from the 557th Field Artillery Battalion in firing position. Morteau, France, November 1944.

Significantly more mobile than towed guns, the M12 battalion received objectives typical for corps artillery. Often, firing was done only on map coordinates, corrected by forward observers, including aircraft-borne ones. Since the appearance of such aircraft was immediately met with AA gun fire, the battalion spent a lot of effort on suppression of such targets. Classical counterbattery fire was rare. German field artillery, unlike AA artillery, was not particularly active. A lot of firing at maximum distances was done.

After the completion of fighting in Falaise-Argentan, the 991st Battalion began the third phase of combat. The 7th Corps, consisting of the 3rd Tank, 1st and 9th Infantry Divisions, in addition to the other units of the 1st Army, began pursuing the retreating Germans through France and Belgium. The 991st Battalion was, as before, attached to the 3rd Tank Division. It was the division's only artillery unit with bigger guns than 105 mm. During offensives, towed medium and heavy guns typically fell behind the tankers by about a day's march. Only the M12s could continue to support the division with counterbattery fire and fire at medium and long range, until the 105 mm M7 SPG battalions, which had greatly inferior range, could pull up with the tanks during an offensive. During the maneuver stage of fighting, the M12s had no issue keeping up with tanks and motorized infantry. The only issues were with supplies of fuel and ammunition. 

One of the batteries of the 991st Battalion is firing. The vehicles are propped up on improvised wooden ramps to increase their range.

On September 10th, 1944, the battalion had the honour of being the first artillery unit on the Western Front to open fire on German territory. The target was a crossroads at Bildchen. Like many other barrages, this one was corrected by aircraft.

Reaching Germany had its impact on the manner of warfare. Positional war replaced maneuvers. The 991st Battalion's tasks now included destruction of fortifications on the Siegfried Line. B battery was attached to the 9th Infantry Division. The two others remained with the 3rd Tank Division. From September 15th to 24th the battalion carried out 35 fire missions in direct fire. In 28 cases, their targets were pillboxes (some of them had to be fired on two or three times), in others: fortified houses and artillery observation points. 16 missions were a complete success: the targets were either destroyed or their garrisons surrendered. On average, it took 10 shots to destroy a target.

One of the SPGs of the 991st Battalion, "Adolph's Assassin". Germany, November 1944.

The result of the fire depended on the quality of the fortifications. For instance, pillboxes with 335 mm thick armour were never penetrated. High quality concrete fortifications were also impervious to 155 mm guns. However, if the materials were of lower quality, the results were different. Cases of 2.5 meter thick concrete walls being penetrated were recorded. There were also attempts made to "smoke out" pillbox garrisons with smoke shells, but they were fruitless.

"King Kongs" fired their guns directly to suppress pillboxes. Luxembourg, February 1945.

In the majority of cases, individual SPGs were sent to knock out fortifications. In Soffer's opinion, the optimal range for firing was 1000-2000 yards. At this range the loss in precision and velocity was not high, but the crew was protected from small arms. The necessity of close cooperation with tanks and infantry that were assaulting the pillbox was highlighted. While the SPG was firing for effect, they were supposed to keep the pillbox suppressed.

M12 battalions typically fought in the aforementioned way in the winter and spring of 1945, typically assigned to tank or infantry divisions. In exceptional cases they were used to solve corps level tasks, such as during the crossing of the Rhine in 1945.

The M12 SPG performed well during the last stages of WWII. It was a powerful counterpart to divisional artillery, which could complete a large spectrum of objectives, from isolating a section of the battlefield to destroying fortifications. Since the M12 used an obsolete weapon its service life was relatively short. In May of 1945 the SPG was classified as of limited combat value, and in August it was deemed obsolete. However, the ideas that the M12 was built with lived on. The T83 prototype SPG was built in February of 1945. The concept was overall similar to the M12, in using the M4 tank chassis and the M2 "Long Tom" gun. After successful trials, the SPG was standardized under the index M40. The T89 203 mm self propelled howitzer was built on the same chassis, standardized as the M43. 

Original article by Andrei Haruk.


  1. I recall a passage from Patton where his troops when confronted with German towns with connected buildings, how they would use mobile 155mm guns to fire a round which would blast a single hole through every building on the block allowing our troops to run through instead of being forced to take the buildings from the street. I assumed Patton was talking about the M-12.

    1. Not a lot of alternatives, were there? Unless he'd gotten some T83 protos for field testing.

  2. I've read similar report of using an M12 near the front of a tank assault on a fortified town. The M12, maybe the 6th vehicle or so in the column, would pull to the side onto the pavement and send a solid AP round, not HE, down the street at waist height and parallel with the street, aimed to clip the face of the row of houses all along that street. It would tear holes along all the houses, allowing GIs to gain rapid entry, concealed by a cloud of dust, and would stun the shit out of anyone inside the rooms. 1 shot, 10 holes open up, storm troops go in.

  3. I am pretty sure that is what the 557th field artillery did in the Battle For Brest. The street fighting was so fierce on the street that the M-12's would fire holes through the side of the buildings so the infantry would ente the side of the buildings.