The idea for the M12 was first proposed in 1941 and the pilot - T6 GMC - built and tested in early 1942. The Army Ground Forces initially rejected the design as unnecessary, but after the Artillery Board supported the Ordnance Department, 100 were authorized and built. These vehicles were at first used for training.
Description
The M12 was built on the chassis of the M3 Lee tank. The engine was moved forward to the center of the vehicle to allow room for the gun mount, and most vehicles used later M4-style bogies with trailing return rollers. The armored compartment at the front was occupied by the driver and commander. The gun crew were located in an open-topped area at the back of the vehicle. It mounted a 155 mm gun M1917, M1917A1 or M1918 M1, depending upon availability, a weapon derived from the nearly identical French 155 mm GPF gun of World War I vintage. Limited storage space meant that only 10 155 mm projectiles and propellant charges could be carried on the vehicle. An earth spade at the rear was employed to absorb recoil. This layout was the pattern adopted for many years by other pieces of heavy self-propelled artillery.
Production
Only 100 vehicles were built: 60 in 1942 and a further 40 in 1943.
Month
M12
September 1942
1
October 1942
37
November 1942
12
December 1942
10
January 1943
16
February 1943
19
March 1943
5
Total
100
M30 Cargo Carrier
Given the limited ammunition carried in the M12, a support vehicle based on the same chassis was produced as the Cargo Carrier M30 to transport the rest of the gun crew and additional ammunition. Identical to the M12 except for the gun and recoil spade, it could carry 40 rounds of 155 mm ammunition, and was armed with a.50-caliber Browning M2 machine gun. in a ring mount for self-defense. In operational conditions, the M12 and M30 would serve in pairs.
Use
During 1943, the vehicles were used for training or put into storage. Before the invasion of France, 74 M12s were overhauled in preparation for combat operations. They were assigned to six armored field artillery battalions and employed successfully throughout the campaign in North-West Europe. Although designed primarily for indirect fire, during assaults on heavy fortifications, the M12s were sometimes employed in a direct-fire role, such as in the Allied assault on the Siegfried Line, where the M12 earned its nickname "Doorknocker" thanks to the 155mm cannon's ability to pierce seven feet of concrete at ranges up to 2,000 yards . The vehicle was also dubbed "King Kong" by American operators due to the raw power of its gun. In 1945, the M12 was complemented in Europe by the M40 Gun Motor Carriage, designed on a late-war M4A3 Sherman chassis. Postwar, the M12 was retired from service and replaced by the M40.
