American wheeled self-propelled guns 75 mm Gun Motor Carriage T27
Experimental prototype of the half-track self-propelled gun 75 mm GMC T12
In the early 1940s, the US Army worked with several contractors to develop new self-propelled artillery units to combat tanks enemy. Various experiments were conducted and experimental vehicles of various designs were tested. One of the most interesting examples of this kind was the wheeled combat vehicle 75 mm Gun Motor Carriage T27.
Artillery evolution
In early 1941, specialized organizations from the US ground forces began developing promising anti-tank self-propelled guns. It was proposed to create new types of combat vehicles by mounting existing guns of different types on available chassis. This approach to design made it possible to achieve the desired result, speed up development, and simplify future production.
By mid-year, the first experimental self-propelled guns of the new types were brought out for testing. These vehicles were built on different wheeled chassis and had guns of 37 and 57 mm caliber. In general, they met expectations, but were criticized for the lack of full-fledged all-round protection, limited driving characteristics and low load capacity.
Soon, an idea emerged to replace the wheeled chassis with a half-track like the M3 armored personnel carrier. Such a vehicle had all the qualities that the existing prototypes were criticized for lacking. Already in early autumn, the new proposal was implemented in the 75 mm Gun Motor Carriage T12 project. A pedestal mount with a 75 mm M1897A4 cannon was placed on an affordable APC.
In September 1941, the first batch of T12 vehicles underwent testing at the Aberdeen Proving Ground. In addition, several dozen of these self-propelled guns were sent to the Philippines for experimental military use. Later, after Japan's attack on the United States, these vehicles took part in combat operations and demonstrated their characteristics in practice.
M1897 gun in towed version
Objective shortcomings
The 75 mm GMC T12 demonstrated high technical, combat and operational characteristics in general. The M3 chassis provided the desired level of mobility on different terrains, and the M1897A4 gun provided high firepower and allowed it to fight almost any existing tanks of a potential enemy.
At the same time, there were some shortcomings. Thus, the half-tracked APC was more complex and expensive to manufacture and operate than any wheeled chassis with similar load capacity. In addition, the T12 SPG was considered too high and visible to the enemy.
A proposal was made to transfer the M1897A4 gun and install it on a wheeled chassis for further testing and evaluation. However, some difficulties arose in finding a suitable platform. The fact is that the wheeled chassis of the production models had insufficient characteristics or were too high for the SPG to have an acceptable height.
New platform
The solution came from the Army Quartermaster Corps. At that time, it was looking for new vehicles and chassis to implement into the existing logistics circuits. The corps tested various models of equipment created by its order or at the initiative of the developers.
Since the summer of 1941, an unusual chassis designed and built by Studebaker had been tested at Holabird in Maryland. It was a two-axle, wheeled vehicle of reduced height with the ability to mount various superstructures and loads. Apparently, this model did not satisfy the quartermasters, and they decided to transfer it to the developers of the self-propelled artillery.
In addition, Studebaker received an order for a second similar chassis. It did not build the same vehicle and presented a platform of a modified design. Like its predecessor, it had a minimum height and could carry various loads. At the same time, the vehicle was significantly redesigned taking into account the new role of a weapons carrier.
The first experimental self-propelled gun 75 mm GMC T27
The first chassis received its gun mount in October 1941 and was then sent to Aberdeen Proving Ground for testing. A second vehicle with a 75mm gun soon followed. Both were designated 75mm GMC T27, despite the differences between the two chassis.
Two modifications
The two experimental T27 SPGs had a similar architecture and were unified in their main units. In both cases, a two-axle wheeled chassis was used without a distinct hull-body and with an open platform of maximum size. In the center of such a chassis was a pedestal mount with a 75-mm gun.
The first chassis was built on the basis of a frame with curved side members, due to which it was possible to reduce the overall height of the car. The main part of the frame was given over to the placement of a platform with all the necessary units, and the engine compartment with a rectangular casing and a rear radiator grille was placed in the stern.
The experimental vehicle was equipped with a gasoline engine and a mechanical transmission that transmitted torque to all wheels. The chassis had a pair of axles with a spring suspension. The exact running characteristics of such a chassis are, unfortunately, unknown.
On the left side of the platform was the driver's workplace with two control posts. In front of the driver's seat and behind it were two steering wheels and separate sets of pedals. This ensured full and convenient control when moving forward and backward without the need to turn the entire machine.
3/4 view of the port side and stern
The second chassis from Studebaker had a different design. The engine compartment was moved to the starboard side and the transmission was rebuilt accordingly. The freed space in the stern was supposed to accommodate one of the crew members and/or the necessary devices. The wheels became double-tired, and fenders appeared above them.
The second experimental platform retained the characteristic dual control post. At the same time, the new project took into account ergonomic issues. The vehicle received additional seats for the crew, which were absent on the previous prototype.
Both versions of the SPG used the same gun mount. A pedestal with gun aiming mechanisms was mounted in the center of the platform. At the base of this mount was a stowage for unitary 75-mm rounds. The aiming drives and sighting devices were borrowed from the M1897A4 gun in its original configuration.
The M1897A4 gun was a light field gun of French manufacture. It had a rifled barrel of 75 mm caliber and 36 calibers. The breech was eccentric, Nordenfelt system. The gun used unitary shots of 75x350 mm R dimensions with projectiles weighing 5,4-7,25 kg.
Depending on the type of shot, the initial velocity of the projectile reached 500 m/s. The maximum firing range was 10-11 km. The rate of fire during prolonged shooting was no more than 3-4 rounds/min.
Predictable outcome
Tests of two versions of the 75 mm GMC T27 SPG were conducted at the end of October 1941. Under proving ground conditions, both vehicles quickly demonstrated their strengths and weaknesses. However, the test results were considered unsatisfactory, and further development of the project was abandoned.
The second experimental chassis, left-top view
It became clear almost immediately that both experimental chassis had insufficient load-carrying capacity. The overweight self-propelled guns demonstrated acceptable running characteristics on a good road, but on the ground they lost mobility and maneuverability. The presence of dual control had virtually no effect on the actual parameters.
Problems also arose with the gun. The 75 mm gun turned out to be too powerful for the new chassis, and both vehicles swayed heavily when firing. In the long term, this also threatened the integrity of the structure. A proposal was made to equip the SPG with coulters to direct the recoil impulse into the ground, but such a device would seriously worsen the combat qualities.
Following short tests, the army decided to abandon further development of the two versions of the T27 SPG. Tests and attempts to correct the identified deficiencies were completed no later than November-December 1941. The project was officially closed only in April of the following year.
Positive result
The 75 mm GMC T27 project ended in failure. Both versions of the wheeled self-propelled gun based on experimental chassis showed low performance and were abandoned. The same thing happened with a number of other experimental wheeled self-propelled guns of that time. Only a few similar vehicles showed sufficient performance and were able to pass beyond the testing grounds.
At the same time, the failure of a number of projects helped the development of the entire self-propelled artillery field. It showed which concepts should be developed and which were not worth attention. As a result, SPGs based on half-track chassis were developed, and the wheelbase was essentially abandoned.
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