M12 Gun Motor Carriage

David Doyle

• Publisher: Pen and Sword Military
• Year: 2018
• eBook ISBN: 9781526743534
• Topic: History
• Subtopic: Military & Maritime History
• Index: History

Although only 100 examples were produced, the 155mm Gun Motor Carriage M12 served with distinction as an infantry-support weapon and in particular as a bunker-buster during the U.S. assault on the Siegfried Line in the winter of 1944-45.The ability to rapidly ready heavy artillery for firing lead to the development of the M12 155mm gun in 1941, based on the M3 Medium Tank chassis. The trial vehicle, built by Rock Island Arsenal, was designated T6\. To accommodate the large, rear mounted weapon – a French-made M1917 155mm gun, the engine was relocated forward to a position just behind the driving compartment. A hydraulically operated spade mounted at the rear of the vehicle, which was lowered during firing to anchor the vehicle against the recoil. When retracted into the travelling position it provided a seat for two of the six crewmen.The production run was between September 1942 and March 1943, with a total of one hundred units being built. Though a few were used by training units, most were placed in storage until February 1944, when the first of 74 of them were shipped to Baldwin Locomotive Works to be improved based on further tests and usage in training. This work continued until May 1944.Three slightly different WWI surplus weapons were mounted depending upon availability, the M1917, the M1917A1 and the M1918M1. The M1917 was French built, the M1918 was US built, and the M1917A1 had the French gun tube, and the US breech.The M12 would earn the nickname "Door Knocker" for its pounding of the German Siegfried Line.

Chapter 1

The T6 Prototype

In June 1941 the US Army authorized the production of a pilot vehicle to be designated 155mm Gun Motor Carriage T6. Work on the pilot began the following month at the Rock Island Arsenal in Illinois. The vehicle mounted a 155mm Gun M1918M1 and its recoil mechanism and top and bottom carriages on a converted M3 medium tank chassis. It was completed in January 1942 and delivered to the Aberdeen Proving Ground for testing. (Patton Museum)

The T6 was assigned US Army registration number 307053. The 155mm gun lacked a gunner’s shield. The barrel rests on a travel lock mounted on the front plate of the driver’s compartment. A side escape door was on each side of that compartment. (Patton Museum)

The side of the left sponson of the T6 was taller to the rear to provide added protection to the two crewmen who were seated there. To the rear of the hull was the hydraulically-operated trail and spade, which transmitted recoil forces to the ground during firing. (Patton Museum)

A GI stands next to the T6 to provide a sense of scale. The driver and assistant driver each had a direct-vision door with a vision block. The armored cover of the final drive assembly is the three-piece type associated with M3 and very early M4 medium tanks. (Patton Museum)

The spade of the T6 is seen in the raised position from the rear. During tests at Aberdeen Proving Ground in March 1942, the hydraulic lifting cylinders of the trail and spade failed due to firing shock and it was necessary to redesign the lifting mechanism. (Patton Museum)

The 155mm Gun M1918M1 was mounted in the rear of the T6, with the rear of the breech to the rear of the hull. A movable platform that slid back and forth on roller bearings was extended to the rear during firing for the crew to stand on. (Patton Museum)

The driver’s compartment of the 155mm Gun Motor Carriage T6 had an overhead hatch on each side and a side door on each side. To the rear of the driver’s compartment was the engine compartment containing a Continental R975-C1 nine-cylinder air-cooled radial engine; the roof and grilles of the engine compartment are visible from this angle. In the right and left sponsons were fuel tanks, with filler caps visible on top. The 155mm Gun M1918M1, its recoil mechanism, its top and bottom carriages, and the elevating hand wheel on the side of its top carriage are in view. (Patton Museum)

The T6’s running gear was the same as the M3 medium tank, with three bogie assemblies on each side, each with two wheels with size 20x9 rubber tires and a track return roller at the top of the bogie bracket. At the rear was a size 22x9 idler wheel. (Patton Museum)

In a view of the T6 from the rear with the spade lowered, the gun and its recoil assembly are mounted on the upper carriage, with a deep U-shaped cut-out to allow for elevating the gun. The upper carriage rotated on the fixed bottom carriage, bolted to the floor. (Patton Museum)

The 155mm Gun M1918M1 on the T6 is at maximum elevation and maximum left traverse. The gun had a maximum elevation of 30 degrees, depression of 5 degrees and traverse of 14 degrees to the right and left of the longitudinal centerline of the vehicle. (National Archives)

To the left rear of the fighting compartment of the T6 was seating for two crew members. There was also seating for two crewmen on the raised spade and for two men in the driver’s compartment. A good view is available of the recoil mechanism of the gun. (Patton Museum)

The operating mechanism of the spade included a hydraulic hoist on each outboard side, with two hydraulic tipping cylinders between the hoists. This mechanism proved trouble-prone during tests; for example, the hydraulic lines were repeatedly severed. (Patton Museum)

The bottom carriage was bolted to the floor of the T6 and provided a mount on which the upper carriage and gun traversed. On the inner face of the raised front of the unit was a traversing sector which engaged with the traversing gear of the top carriage. (Patton Museum)

The top carriage, as seen from the front, had a trunnion bearing on each side of the top, an elevating hand wheel on the left side and a traversing worm gear on the lower front. The traversing hand wheel is mostly hidden to the rear of the elevating hand wheel. (Patton Museum)

As seen from the rear of the top carriage, the nearer hand wheel is the traversing and the one to the front of it is the elevating. Inside the bottom of the carriage is the elevating worm gear, which engaged the elevating quadrant on the bottom of the recoil mechanism. (Patton ...

Table of contents

• Cover
• Dedication
• Title
• Copyright
• Contents
• Acknowledgements
• Introduction
• Chapter 1: The T6 Prototype
• Chapter 2: The M12
• Chapter 3: The T14
• Chapter 4: The M12 in Combat
• Chapter 5: The M12 Preserved
• Notes and References