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T3 HMC: America's First (Proper) SPG
History:During the 1930s, the US Military began to go through a massive re-armament program. Part of this program was to design and adopt a whole slew of new armored vehicles for the Infantry and Cavalry. One of the issues which became apparent during this rearmament program was the internecine fighting over which element of the Army controlled the tanks. Because of this, the Cavalry was forced to call theirs "combat cars", while in reality it was the exact same thing as a "tank" in Infantry service. The very first of these combat cars was the M1, a very simple vehicle mounting a .50 Browning and a .30 Browning machine gun in a single turret. This vehicle would serve the Cavalry in limited numbers, primarily as a training vehicle, as it never saw service with the US. However, it would not only serve as a light combat car.
The US Army also needed a self-propelled gun which could destroy heavier targets than the machine gun-armed tanks and combat cars could engage. That is where the T3 HMC steps in. Attempts had been made in the past to develop a vehicle armed with a large-caliber gun, such as the T1 HMC and M2 GMC, but these were dismal failures. The T3 HMC was somewhat more successful, though it too would remain a prototype. The idea was to use an existing vehicle as a base, thereby reducing the load on factories and logistics, and the chosen vehicle was the M1 Combat Car. It would be redesigned into a casemate-style SPG with the 75mm Pack Howitzer M1 as its main armament. The turret from the M1 CC would be removed and replaced with the .30 caliber turret from an M2A3 light tank as a secondary weapon. The resulting vehicle was somewhat similar to the SAu 40 in concept, with a 75mm gun in the hull and a .30 caliber machine gun in a defensive turret. Initial trials showed that while good in concept, there were many flaws. The gun was much too large for the chassis, even with the redesigned casemate housing. As a result, only two crew could reasonably fit in the vehicle, though on paper the vehicle could carry three. This caused the rate of fire and accuracy to be abysmal, as the commander had to operate nearly every aspect of the vehicle that didn’t involve driving. Mobility was shown to be decent enough, though the acceleration was slower due to the weight of the gun and its ammo. The gun also had to have special hatches opened from the outside in order to aim left and right independent of the vehicle. This gave it a 25-degree arc of movement, but opened up a massive weak spot with no armor whatsoever. It was near impossible to fire on the move, and the fact that the commander was also loading, aiming, and often firing the main gun in addition to the machine gun turret and his normal commanding duties made this even more of an impossibility. With Aberdeen, Fort Bragg, Fort Knox, and the 3rd Army all agreeing, the project was cancelled in June of 1941. The lessons learned by this project would help the future of SPG design in the US, with another project known as the T18 HMG being developed immediately afterwards, though it too would not succeed. It wouldn’t be until the T30 HMC and M8 Scott that the 75mm howitzer carrier so desperately wanted would finally be available.
Description:Being based on the M1 Combat Car meant that this was a very mobile and lightly armored vehicle. At most it had 16mm of armor, with 6mm at its thinnest. Powered by a Continental R-670 7-cylinder gasoline radial engine, it could reach a maximum speed of 72 km/h (44 mph). The turret on the roof held an M1919 Browning .30 caliber machine gun for defense and was manually operated by the commander. The driver was theoretically the gunner as well, but in most cases the commander took this duty. Ideally there would be a third crew member in the final model, but this was seen as impossible. As a result, I think this vehicle should have two crew members, as that is all it was ever tested with. The front doors which allowed the gun to move horizontally could be optionally opened or closed like the doors on the VFW. This would allow for an option to have better protection or more gun movement. Ammunition would be the same as on the M8 Scott, with HE, HEAT, and Smoke. 61 rounds of 75mm ammunition are carried, along with 3600 rounds of .30-06 for the machine gun.
Main Armament: 75mm Pack Howitzer M1
Secondary Armament: 7.62mm M1919 Browning machine gun (turret on roof)
Armor: 16mm maximum, 6mm minimum
Ammo Count: 61 75mm shells, 3600 machine gun rounds
Engine: Continental R-670 7-cylinder gasoline radial, 250HP
Transmission: 5 forward, 1 reverse gear
Top Speed: 72 km/h (44 mph)
Gun Movement: 75mm: +20/-10 elevation and depression, 10 degrees left/15 degrees right (with doors open); Machine gun: +20/-10 elevation and depression, 360 degree traverse
Crew: 2/3 (Driver/Gunner, Commander/Gunner/Loader/Machine Gunner, Proposed Gunner/Loader (not tested))