Even at the final stage of the First World War, American engineers created an automatic propelled gun based on the French 155-mm GPF cannon designed by Captain Philois (US Army designation M1917 / 18) and one of the tracked tractors available. This development made it possible to slightly increase the mobility of artillery on the battlefield, but still did not solve the whole range of tasks. In the twenties and thirties, work in this direction continued, but also did not result in much success. Difficulties of a technical nature were often combined with administrative problems. As a result, the concept of 155-mm guns on a self-propelled chassis was not developed, and artillery units continued to use towed guns and horse traction.
The situation changed only in 1940. News from Europe forced the US military to change their views on the material part of the troops. Successful German operations in Poland and France showed the importance of mechanized troops and self-propelled vehicles. In this regard, the US military department decided to begin upgrading artillery formations, the purpose of which was to abandon horse traction and a gradual transition to tractors and self-propelled artillery installations. In particular, the existing 155-mm M1917 / 18 guns were proposed to be used as part of an ACS.
In 1940, a new M3 Lee tank was developed in the USA, which was distinguished by a rather successful tracked chassis and powerful weapons, which included two guns and machine guns. This development interested the artillery command, which resulted in a proposal regarding the technical appearance of a promising self-propelled gun with a 155-mm gun. According to the new proposal, the new armored car was supposed to have a chassis tank M3 and equipped with a gun M1917 / 18. This made it possible to simplify the development of self-propelled guns, as well as to find application for old guns that had good characteristics, but did not differ in high mobility.
The new project received the designation T6 Gun Motor Carriage (T6 GMC) and retained this name until its adoption. The design work started at the end of 1940 and lasted until the summer of 41. In June 1941, the Department of Arms of the US military commissioned the Rock Island arsenal to produce a prototype of a promising self-propelled gun. Due to the presence of other orders and the loading of available capacities, the order was executed only in the winter of next year. The prototype T6 GMC was rolled out of the assembly workshop in February of the 42.
The project of self-propelled gun T6 was of great interest for the army, but its development and refinement was associated with some difficulties. Despite the use of the finished chassis, some of the components and assemblies had to be developed from scratch. Later, some new details did not work out for the best. In particular, problems with hydraulics could deprive the car stroke. Nevertheless, the work continued until the complete solution of all problems.
As a basis for a promising SPG, the chassis of the M3 Lee tank was taken, which had to be significantly reworked. At the front of the base tank, there was a part of the transmission units connected to the aft engine-transmission compartment with the help of a cardan shaft. Above the shaft was located fighting compartment. Because of the need to free up the volume in the aft hull, which was proposed to be used to install 155-mm guns, the engine had to be moved to the middle part of the hull. In this regard, the drive shaft was shortened and located at a different angle to the horizontal. Together with the engine, several fodder fuel tanks “moved” to the middle part of the hull.
The upper part of the hull, located above the tracks, has undergone dramatic changes. Due to the movement of the engine and the need to maximize the reduction of the line of fire, a new upper hull was designed. The original inclined frontal sheet was replaced with a smaller part located at a small angle to the horizontal. Behind it was an inclined sheet with inspection hatches of the department of management. The middle part of the body was made in the form of a box-like structure with a horizontal roof and ventilation grilles.
On each side of the main body of the hull, on the futon shelves were placed boxes for the transport of ammunition and other property. In the stern of the car, on the site of the former engine compartment, there was a large niche, the floor of which was located below the level of the fenders. On each side of it covered several plates with additional boxes.
The body was proposed to be assembled from armor plates with a thickness from 12,7 to 50 mm. The thickest parts were located in the frontal part of the body and were supposed to protect the car from shelling from the front.
The power plant and transmission of ACS T6 almost did not differ from the corresponding units of the base tank M3 Lee. The Wright (Continental) R975 EC2 HP 350 radial petrol engine is located in the middle of the hull. With the help of a cardan shaft passing through the control compartment, the engine was connected to the transmission units, which provided the rotation of the front drive wheels. With the exception of the shortened drive shaft, the transmission was unified with the base tank.
Chassis T6 GMC has retained six road wheels on each side. The rollers were interlocked in pairs and equipped with a spring suspension. Support rollers were provided at the top of the carts. The drive wheels of the hopper gear were located in the front of the hull, the guides were in the stern.
Calculations showed that the existing undercarriage could not hold the self-propelled gun in place when fired from a powerful X-gun caliber 155 mm. Because of this, it was supplemented with a set of special tools. At the rear of the hull, a coulter with hydraulic cylinders was placed for raising and lowering. In addition, before shooting under the front of the tracks it was proposed to put special shoes.
On the stern of the hull located tumbovnoy installation with U-shaped fasteners for guns. The installation design made it possible to direct a weapon within the horizontal sector with a width of 28 °. Vertical pickup angles range from -5 ° to + 30 °. As part of the artillery, the M1917 / 18 gun with the M4-type cradle was used. For targeting guns were proposed telescopic sight M53, panoramic M6, as well as artillery quadrant type M1918A1.
The M1917 family was equipped with a 155 mm caliber barrel with a 36,4 caliber length. The full length of the gun exceeded 6 m. The gun was equipped with a screw bolt, when opened, shifted to the right. Used separate loading. The ammunition consisted of three types of ammunition: high-explosive fragmentation M101 weighing 42,96 kg, armor-piercing with an additional explosive charge M112BX1 weighing 42,36 kg and smoke M104 weighing 44,5 kg. Shells were used with a standardized liner with a propellant charge. In the fighting compartment of the ACS T6, the entire 10 shots were placed separately loaded. Part of the ammunition was fixed on the floor of the fighting compartment, the rest were located in the side boxes.
The M1917 / 18 gun had rather high characteristics. At the optimum elevation angle, the high-explosive projectile flew to a distance of 18,4 km, and the armor-piercing from the 1000 distance yards (914 m) at a meeting angle 60 ° punched up to 119 mm of homogeneous armor. Such characteristics of the weapon in theory allowed the destruction of armored vehicles and the strengthening of the enemy by direct fire from a distance of about 1-1,5 km. In practice, fire was often fired from long distances.
General view of the updated fighting compartment. Figure Alternathistory.com
The self-propelled crew consisted of six people. The driver and his assistant were to be located in the control compartment inside the hull. Four more crew members in battle and on the march were in the fighting compartment. Their task was to maintain weapons and firing at the enemy. The project T6 did not provide additional weapons for self-defense. For these purposes, the crew had to use its own M1 carbines, as well as hand and rifle grenades.
The means of internal and external communication had an unusual composition. Radio station or intercom for the crew were absent. At the same time, however, there were a telephone and a set of signal flags to communicate with other crews in the firing position during joint combat work.
Due to the use of the original opener and the old implement, the promising self-propelled gun T6 differed from the M3 tank in terms of both dimensions and weight. Its total length was 6,77 m, width - 2,68 m, height - 2,88 m. The combat weight reached 26786 kg. The existing power plant made it possible to reach a speed of up to 38 km / h on the highway, the fuel supply was enough for 192 km of the way.
Prototype conveyor T14 / M30. Photo Afvdb.50megs.com
The first prototype of the T6 ACS was built in February 1942. Soon his tests started, the platform for which was the Aberdeen Proving Ground. The first checks did not reveal any noticeable problems of the power plant, chassis or armament. Nevertheless, the drawbacks of the opener and its hydraulic drive appeared. After one of the shots, the hydraulic cylinders failed, due to which the coulter hit the ground was blocked in the lower position. Self-propelled gun lost the ability to move. For this reason, it was necessary to recycle the design of the entire stop device, both the vomer and its drives. In the future, this site again underwent modifications aimed at simplifying the design and improving reliability.
Correction of the opener deficiencies allowed to continue the tests, which ended in success. Soon, the second stage of inspections was to start, during which disputes about the future of the new ACS began. The Armaments Department offered to order about 50 new combat vehicles without waiting for the end of their tests, but the leadership of the military department demanded that all necessary checks be carried out and only after that should they make a final decision.
Until May, 1942, the American experts were engaged in additional checks promising ACS, and the emphasis was placed on the work of the power plant and chassis. In terms of mobility, the T6 GMC differed little from the tank M3 Lee. At the same time there were significant advantages over towed artillery. During one of the checks, the T6 crew "took off" and moved to another position in 6 miles from the original, spending minutes on all 35 operations. Tracked tractor with towed gun M1917 / 18 was able to perform a similar task in only three hours.
According to the results of the second stage of testing, it was recommended to once again refine the structure of the opener, improve the pickup systems, optimize the ergonomics of the fighting compartment and make some other minor changes. The elimination of the last drawbacks took just a couple of months, thanks to which, in July 1942, SAU T6 was recommended for adoption. In late August, it was adopted under the designation 155 mm Gun Motor Carriage M12. By this time, Pressed Steel Car was already in the process of completing the first T6 / M12 serial construction order.
Simultaneously with the adoption of SAU in service, it was recommended to develop a special transport of ammunition on a similar chassis. This machine has received the working designation T14. The ammunition transporter was carried out on the basis of the original T6 project. Moreover, the prototype of this machine was made from the first prototype self-propelled guns. An instrument was removed from its combat compartment, and a racks for ammunition were installed instead. The size of the fighting compartment allowed to place 40 shells and the same number of shells with a propelling charge. On the sides of the stern T14 installed ring turret for machine gun M2, proposed for self-defense. Instead of a coulter on the stern of the conveyor hull, a tailgate with a winch for lifting was installed. The T14 machine was slightly shorter than the base T6 and was distinguished by a lower combat weight - 20,7 t.
The first order for the construction of the 50 ACS M12, placed in the middle of summer 42, was completed by the middle of autumn. By this time, Pressed Steel Car had time to begin production of T14 transporters, adopted under the designation M30 (according to some data, this index appeared only in 1944 year). In August, a second order for 50 self-propelled guns appeared, which was carried out until the spring of 43. Production of ammunition transporters was completed in the same period, but their exact number is unknown. According to various sources, it was manufactured from 70-75 to 90-100 machines. In the latter case, each self-propelled gun could get on its own transporter.
SAU M12 GMC is firing. France, 1944. Photo Militaryfactory.com
The start of the M12 self-propelled gun service was unusual. Several serial cars were sent to the Erie and Fort Bragg test sites, where additional checks and training of future crews were conducted. Only a few SAU managed to get to the landfill. Others were less fortunate: in accordance with the order of the command, they were sent directly from the factory for storage. This decision was officially explained by the need for additional training of personnel.
The M12 GMC self-propelled artillery mounts were recalled only at the end of the 43. By this time, the American command received detailed information on the use of large-caliber artillery on the battlefields in Europe, which influenced his plans. It was decided to use M12 in the upcoming landing in Normandy. Began additional checks to establish the status and prospects of technology.
It turned out that the latest self-propelled guns have already become obsolete. The army was in full swing mastering the M4 Sherman medium tanks and gradually abandoned the M3 Lee. Tanks "Li", in turn, should not have fought in Europe, which could cause problems with the supply of self-propelled guns spare parts. After analyzing the pros and cons of existing technology, as well as its prospects in future battles, it was decided to refine M12 to improve performance and provide some unification with the front-line equipment.
Alteration of the ACS started at the very end of 1943 of the year. The coulters of the cars lost their hydraulic drive: it was proposed to raise and lower them with a simple and reliable manual winch. The opener support beams for greater convenience were connected by a metal ladder. Trolleys with twin support rollers of the M3 tank were replaced with similar units of the new Sherman. Before sighting devices appeared 19-mm armor shield. Also, the types of some additional equipment were changed, which in some cases led to a re-assembly of the units. Finally, the car received U-shaped mounts for an awning.
Six divisions armed with M12 GMC were to take part in the upcoming landing in Normandy. For their weapons ordered the modernization of 75 serial machines: four for each of the three batteries of the division, plus three standby ones. The works started in February of 44 and ended in May. For some reason, upgrades were 74 machines, which, however, allowed to complete all the connections. By the end of July, self-propelled guns were shipped to Europe.
The 72 ACN MXXUM with M12 ammunition transporters were combined into six separate divisions, which were attached to the hulls if necessary. The firepower and mobility of the vehicles allowed these units to participate in both offensive and defensive operations. It was possible to directly support the troops with direct fire and shooting from closed positions, including the counter battery. Often, gunners had to destroy the enemy's long-term firing points. Concrete structures were effectively destroyed from a distance of no more than 30 m.
One of the surviving samples of the ACN M12. US Army Ordnance Training and Heritage Center, Aberdeen. Photo Afvdb.50megs.com
Six separate divisions mostly fought in France. For several months of fighting, the crews of self-propelled guns had time to work out the effective combat use of technology, as well as to work out some new techniques. For example, to increase the range of fire in military workshops, special wooden inclined ramps were made. Entering such a design, the M12 GMC increased the actual elevation angle of the barrel and due to this it could attack targets at a greater range than the tool pointing mechanisms allowed.
After the start of participation in the battles self-propelled M12, like many other types of equipment, received an unofficial nickname. For the high power of the gun, it was nicknamed “King Kong”, in honor of the famous movie hero, who was distinguished by gigantic size and enormous power.
M12 self-propelled artillery mounts with M1917 / 18 guns with various successions were used in the West European theater of operations until the very end of World War II. Shortly after the victory over Germany, the career of these machines was over, which was facilitated by the moral and physical obsolescence of equipment and weapons. In May, the command of the 1945 of the year recognized the M12 ACS with limited use, and at the end of the summer it was called completely obsolete. The basic chassis and propulsion of the M3 Lee tank, even after the introduction of individual elements of the M4 tank did not meet modern requirements. Similar problems were in the gun during the First World War.
Despite the fairly high combat performance, self-propelled M12 GMC no longer met the requirements of the military and could not solve the tasks. Already at the beginning of 1945, the T83 ACS was developed based on the Sherman tank with the M2 gun, which was based on the same ideas as in the case of the T6 / M12. A little later, it was adopted under the name M40 GMC. M12s that have become unnecessary have been decommissioned, decommissioned and disposed of. Only a few samples have survived, which are now museum exhibits.
On the materials of the sites: