The M12 155mm Gun Motor Carriage was one of the heaviest self-propelled guns to see service with the American Army during the Second World War, but its development was delayed by Army Ground Forces, the command responsible for developing the army in the United States.
During the inter-war period the US Army's Field Artillery Branch remained convinced that towed artillery would be usable on the modern battlefield, using either horses or heavy tractors. After the outbreak of the Second World War, and in particular the rapid German victories in Poland in 1939 and in the west in 1940 the Artillery Branch began to change its mind.
By the summer of 1941 the Ordnance Department already carried out some studies into the possibility of mounting a 155mm gun on the chassis of the M3 Lee medium tank. In June 1941 the Ordnance Department suggested that work should begin on a pilot of this vehicle. The Artillery agreed, and work began on the prototype T6 155mm gun motor carriage.
The T6 was armed with the M1918Ma 155mm gun, which was based on a French design of the First World War. The recoil mechanism, top and bottom carriages of the gun were retained, and were mounted on a modified M3 medium tank chassis. The working parts of the gun were mounted at the rear of the chassis, with the gun crew working on the gun platform. The T6 had a hydraulically operated recoil spade that was designed to stop the vehicle moving backwards when the gun was fired. The vehicle had a driver and a co-driver, located in the front of the vehicle. The engine was moved from the rear of the vehicle to the middle, to make space for the gun.
The T6 prototype was built at the Rock Island Arsenal and went to the Aberdeen Proving Ground for tests on 12 February 1942. The first firing tests were a partial success. The T6 was a stable gun platform, but the recoil spade caused problems. It tended to catch the ground even when retracted, and the hydraulic system broke, preventing it from retracting.
After these problems had been fixed the T6 was sent to For Bragg, North Carolina, where it was tested by the Field Artillery Board. In March 1942 the Ordnance Board asked for production of fifty T6s, but Army Ground Forces objected. General McNair was a great supporter of towed artillery, even believing that it could be used in the anti-tank role. The Ordnance Board disagreed, and so the T6 underwent trials at the Artillery Board.
The T6 excelled in these tests. In once case it took the T6 thirty-five minutes to fire, move six miles then fire again, while a normal towed 150mm gun took three hours. The Artillery Board supported the Ordnance Board and the order for fifty T6s was placed with the Pressed Steel Car Company.
In August 1942 T6 was standardized as the M12 155mm Gun Motor Carriage and the order was doubled to 100 vehicles. This was also accompanied by an order for the same number of M30 Cargo Carriers, which was effectively the same vehicle but with the gun removed. Each M12 operated with an M30 which carried its ammunition supply and half of the crew of twelve.
The final M12 was completed in March 1943. Despite all of the effort that had gone into their production, the Artillery Branch now demonstrated very little interest in the M12 and for most of 1943 they were either mothballed or used in training. In October 1943 the Army Ground Forces declared that it no longer needed the M12, which it had never really wanted.
This chanced in December 1943. Planning for the D-Day invasion was now far advanced, and the Artillery decided that it wanted every weapon that it could get. Seventy four of the T12s were modernized at the Baldwin Locomotive Works, where they were given stronger suspension taken from the M4 Sherman and a small gun shield. This work was completed by May 1944 and the modified M12s were sent across the Atlantic.
By D-Day the M12 had been allocated to six artillery battalions. These units soon moved to the Normandy beachhead, and the M12s took part in the fighting in France. During the rapid breakout from Normandy the M12 was often the only heavy artillery able to keep up with the fast-moving armoured divisions. The M12 also played an important role in the attack on the Siegfried Line, on the German border, where it was often used in the direct fire role to attack German bunkers at relative short range (for a 155mm gun). The M12 could penetrate up to 7ft of reinforced concrete when using the correct shells (with concrete piercing fuses).
The M12 was also used in the battle for Cologne, where it operated alongside the M40 155m Gun Motor Carriage, a similar but improved vehicle that had been designed during 1944.
After the end of the war in Europe the M12 was declared obsolete and the surviving examples were scrapped.
Hull Length: 22ft 1in
Hull Width: 16ft 1/2in
Height: 9ft 10in
Crew: 6 in M12 (commander, driver, four gun crew), six more gun crew in M30
Engine: 353hp Continental R-975
Max Speed: 24mph road, 12mph cross country
Max Range: 140 miles road radius
Armament: One 155mm M1918M1 gun, one .50in Browning anti-aircraft machine gun