M56 Scorpion – Lightweight Self-Propelled Gun

The need for a lightweight self-propelled anti-tank gun was identified in the late 1940s. The T101 development program took just over 5 years and $2.5 million dollars to develop what became the M56.

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Front, right view of the M56 Scorpion (Matthew Moss)

The M56 was intended to act as an airmobile support weapon for the US Army’s airborne forces that was capable of traversing muddy, marshy, sandy and snowy terrain. Airborne infantry have historically been lightly armed and sometimes struggled against enemy forces equipped with armour. Attempts to level the playing field with glider transported anti-tank guns or even super light tanks could only do so much. Airborne operations during world war two proved light tanks, like the M22 Locust, were out-gunned, under-armoured and largely useless. While light artillery proved effective it lacked mobility and while infantry anti-tank weapons like the Bazooka were extremely useful they were close quarter weapons.

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The T101 – Prototype M56 (US National Archive)

In response the US Army decided to abandon one of the points of the classic ‘iron triangle’ of armoured vehicle design all together, sacrificing protection for firepower and mobility. The unarmoured 16,000 lb (7 tonne) vehicle could be dropped from a transport plane to support paratroops and later heliborne air cav units.

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An M56 (US Army)

The M56 was developed and manufactured by the Cadillac Motor Car Division of General Motors, a pilot version was completed by 1955. The pilot model, designated the Full-Tracked Self-Proppelled Gun T101 is seen in this photograph from October 1955. When the vehicle finally entered production in 1957 it was largely identical to the T101 except for changes to the location of the radio, the design of the gun’s muzzle device, the hinged flaps on the gun shield were abandoned and the sand skirts were also abandoned. The Scorpion had no secondary armament and no armour. Protection for its 4-man crew amounted to nothing more than a gun shield, which also has a window for the driver.

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A view of the ‘fighting compartment’ of the M56, the ammunition storage rack is missing(Matthew Moss)
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View of the driver’s position (Matthew Moss)

The 90mm M54 high velocity gun was mounted on a pintle in the centre of the vehicle with the driver on the left and the vehicle commander, loader and gunner sat around the gun. The gun could be traversed 30-degrees left and right and had 10-degrees depression and up to 15-degrees  elevation. At the rear of the vehicle was an ammunition store that held 29 90mm rounds. The main ammunition used with the M54 would have been the M318 armour piercing round but it could chamber any of the other 90mm ammunition then in US service. Sadly the ammunition store and breech of the gun aren’t present on this example.

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An M56 in action, note the ammunition storage racks (US Army)

The gun’s impressive recoil, even with a pair of recuperators, hit the Scorpion and the crew manning it hard, the footage shows just how powerful the recoil was. In this contemporary footage we can see that the front wheel almost bounces off the ground. Note also the semi-automatic action that opens the breech and ejects the spent shell casing after the gun is fired.

The vehicle was powered by an air-cooled petrol engine that produced 200hp. Capable of a maximum speed of just over 28mph.

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Front, left view of the M56 (Matthew Moss)

Perhaps its most interesting feature is that it runs on four pneumatic road tyres, rather than metal road wheels, enclosed in a 20in wide track. The track was made up of two bands of rubber and steel cross pieces. There’s a sprocket at the front and an idler at the rear for tensioning the track. This configuration was chosen to reduce weight and it also minimised the ground pressure of the vehicle.

Production of the M56 ended in 1959, after around 325 had been produced. While most of these entered US service, 87 were purchased by Morocco in 1966 while a further 5 were used by the Spanish Marine Corps.

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M56s in Vietnam (US Army)

While the M56 was slightly more mobile across country, in reality it wasn’t much better than a standard jeep equipped with a M40 recoilless rifle. Despite the Scorpion’s shortcomings it remained in service into the early 1970s and saw action during the Vietnam War with the 173 Airborne Brigade, which had a company of 16 M56s. In Vietnam the more capable and better protected M551 Sheridan saw wider use and what action the Scorpions did see was largely acting in direct fire support.

Special thanks to Battlefield Vegas for allowing us to film and feature their M56.


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Specifications:

Length: 19ft 2in / 5.8m

Height: 6ft 9in / 2m

Width: 8ft 4in / 2.5m

Weight: 7 tons

Powerplant: Continental A01-403-5 gasoline engine

Speed: ~28mph / 45km

Armour: Unarmoured

Armament: 90mm M54 Gun


Bibliography:

TM 9-1300-203, Artillery Ammunition (1967)

M50 Ontos and M56 Scorpion 1956–70: US Tank Destroyers of the Vietnam War, K.W. Estes (2016)

Airborne “Scorpion”, A. Haruk, (source)

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