[Would you like to see this in-game?]
TL;DR: An M37 HMC fitted with a T21 recoilless rifle instead of a .50 cal.
Note: This is the only picture I could find of this tank. If you have any other significant information, please let me know.
History:
The need to motorize artillery was felt by the US military as early as WW1. Many projects attempted to satisfy this need, which was always evolving as time passed and technology improved, most failures but some successful. One such rare success was the M37 HMC, a lightweight and very mobile casemate 105mm howitzer carrier based on the chassis of the successful M24 Chaffee. In 1943, it was becoming apparent that the M3 Priest just wasn’t cutting it. It’s armament was too light for its mediocre speed and the vehicle itself was unnecessarily heavy, however, light artillery was still seen as necessary. As such, developments to mount the 105mm M4 howitzer onto a lighter and more mobile platform began. The obvious choice for this was the developing M24 Chaffee or the T24 Light Tank as it was known at the time. The finished M37 would be based on an extended M24 chassis and would share many design elements with its M7 predecessor, the most prominent of which being the “pulpit-like” machine gunner position, which would typically mount a .50 cal. The M37 would be accepted for standardized production in January of 1945, a tad too late to see service in WW2. In the spring of 1945, the US began tests regarding the mounting of recoilless weapons, which had just barely seen action and results during WW2, onto preexisting motorized platforms. The first of these tests would be simple in nature and would mount a 75mm T21 recoilless rifle in place of the .50 cal. The new weapon was to be fired by the M37’s assistant mechanic. After undergoing firing trials, it was found that the M37’s open-topped nature severely hampered the effectiveness of the T21 its the dangerous backblast and gasses would vent directly over the fighting compartment, which could damage internal components and easily result in fatalities in a stressful combat situation. As a result, safe firing sectors were imposed, however, these sectors were not always in convenient locations for effective fire. It was eventually determined that the M37 HMC and its pulpit were not suitable to carry recoilless weapons and tests moved on.
Place In War Thunder:
Quick derp vehicles have always had their fair share of fans. Multi-gun vehicles have always had their fair share of fans. What happens when you combine the two vehicle types? You get the M37 (T21) HMC, a vehicle with a completely unique armament arrangement when it comes to War Thunder as no current vehicle has access to a conventional cannon as well as a recoilless rifle. Playstyle would, of course, be rather unique as well. Thanks to being based on a M24, you can actually get around rather quickly and potentially show up in places where you aren’t expected. While your main weapon is bound to your casemate, your recoilless rifle is not and could potentially surprise enemies. In addition, since this is War Thunder, the safe firing sectors can be completely ignored, making you capable of defending yourself from any angle. Ideally, you’d want to save the howitzer for heavier targets take anything else on with the recoilless rifle. The best in-game placement for this unicorn is, of course, as an event or premium vehicle. It was a failed one-off testbed that only existed for a short while before being returned to its original form. It has no place in the tech tree.
Specifications:
Armament: 105mm M4 howitzer and 1x 75mm T21 recoilless rifle
Dimensions: 5.49m, 3.00m, 2.77m (L,W,H)
Weight: 20870~kg
Armor: Proof against small arms fire and artillery splinters
Crew: 7
Ammunition:
-105: Same as M4 (105) in-game (plus HESH if necessary)
-75: HEAT and HE
Speed: 56kph
Horsepower: 296hp
Sources:
M20 recoilless rifle - Wikipedia
M37 105 mm Howitzer Motor Carriage - Wikipedia
Tank Archives: A Fast Howitzer on a Light Chassis
Early American SAU projects with recoilless guns
TM 43-0001-28 Artillery Ammunition
Stuart: A History of the American Light Tank Volume I by R.P. Hunnicutt