The Self Propelled 155mm “Doorknocker” or “Bunker Buster” – You Will Not Believe The Holes This Could Punch!

Even though the M12 Gun Motor Carriage was primarily designed for indirect fire when assaulting heavy fortifications, the M12s were at times used in a direct-fire role too. For instance, during the assault on the Siegfried Line and in fighting in the German City of Aachen it was used to great effect.

The M12 was given the nickname “Doorknocker” by the troops because of the ability to punch through seven feet of concrete even when firing from 2,000 yards! “King Kong” was also a favorite nickname with the American operators of the M12, again due to the raw power of the gun. On battery managed to destroy over 50 bunkers in just a few weeks.

The chassis of the M3 Lee was used for the 155mm M12. The armored driver’s compartment was shared with the commander, and the gun crew was located at the back of the vehicle in an open topped area.

This setup left limited storage space which meant that only ten projectiles and propellant charges could be carried on the vehicle. Normally it was accompanied by an extra vehicle to carry extra ammunition.

Just 100 M-12s have been built: 60 of those 1942 and a further 40 in 1943. At first, the vehicles were mostly used for training or put into storage, but 74 M12s were overhauled right before the invasion of France in preparation for combat operations in North-West Europe.

In the summer of 1944, six field artillery battalions were outfitted with the M12 155mm GMC: the 557th, 558th, 981st, 987th, 989th and 991st Field Artillery Battalions.

M12 155mm GMC named “Aiming Post Annie” (registration 4055517/serial no. 16) fires on targets in Germany from its position in Belgium along the Moselle River on 8th September 1944. Chalked on the side of the vehicle are inscriptions from Belgians celebrating their liberation when the US Army drove through one of the neighbouring towns.

M-12 155mm Gun Motor Carriage from 103rd Infantry Division, US Seventh Army, moves up to assault pillbox defenses of the Siegfried Line, Niederschlettenbach, Rhineland-Palatinate, Germany, 20 March 1945.

M-12 155mm Gun Motor Carriage named Corregidor fires on German positions near St. Lo, France, 1944.

The crew of a M12 155mm GMC nicknamed “The Babe” (registration 40781014/serial no. 62) loads a round into the gun during the bombardment of Fort Driant, part of the Metz fortifications, during Patton’s ill-fated campaign around the city on 10th October 1944. The heavily fortified border city of Metz was a thorn in Patton’s side for most of the autumn of 1944.

M-12 self-propelled 155mm Gun Motor Carriage bombarding Gressenich, Germany, 1944.

During the assault toward the Roer River, this 155mm Gun Motor Carriage M12 is firing on German held positions, circa November 1944.

M12 155mm self-propelled Gun Motor Carriage, firing at German positions, ETO, circa March 1945

A M12 155mm GMC nicknamed “Choo-Choo-Bamâ” (registration 4081040/serial no. 88) provides fire support for Patton’s Third Army during fighting in Echternath, Luxembourg on 8th February 1945. This vehicle’s tracks have been fitted with extended end connectors for better operation in mud.

A M12 155mm GMC nicknamed “Adolph’s Assassin” (registration 4081045/serial no. 93) from Battery A, 991st AFAB prepares to fire near Kornelmunster, Germany on 4 November 1944. This view shows the 155mm gun elevated near its maximum. By this stage, the Battalion’s distinctive unit insignia has been painted over.

A M12 155mm GMC (registration no. 4081011/serial no. 59) of Battery C, 557th Field Artillery battalion, near Morteau, France on 15th November 1944. This photo shows the folding chocks placed under the front tracks, and also the hoops for the camouflage net erected over the gun compartment. This vehicle is unusual in that it is still fitted with the front portion of the sand skirts, a feature not commonly seen on M12 155mm GMCs in Europe.

An overhead view over the right side shows typical stowage. This picture was from a series of photos taken in 1943 during a demonstration of the M12 and M30 for the Field Artillery Board.

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 gun crew and additional ammunition.

A M30 cargo carrier of the 557th AFAB moves over a treadway bridge outside Linnich on 26th February 1945. This unit welded rods along the superstructure for crew stowage. This particular vehicle is fitted with extended end connectors for better mobility in the mud.


Sources: 1,2,3

Joris Nieuwint

Joris Nieuwint is a battlefield guide for the Operation Market Garden area. His primary focus is on the Allied operations from September 17th, 1944 onwards. Having lived in the Market Garden area for 25 years, he has been studying the events for nearly as long. He has a deep understanding of the history and a passion for sharing the stories of the men who are no longer with us.