The M6 37mm gun motor carriage was the one of the first, and the cheapest, tank destroyers to see service with the US Army during the Second World War. In 1940-41 US Army Ground Forces saw the tank destroyer as the most effective way to deal with the German blitzkrieg. The first of defence would be the anti-tank gun. The tank destroyers would be kept in reserve and would rush to the site of any breakthrough to deal with any enemy tanks.
US Army Ordnance began to work on a number of ways to mount the standard 37mm anti-tank gun on various light vehicles. Attempts to mount the gun on a jeep failed because of a lack of space on the smaller vehicles and so in the summer of 1941 work began on the T21, which mounted a 37mm gun on a Dodge 3/4 ton weapon carrier (based on the chassis of a 3/4 ton truck).
Prototypes were developed with forward and rearward mountings. The forward mountings were discarded after it was discovered that the blast from the gun could injure the driver and smash the windscreens. The rearward mounting avoided this danger, and also made it easier for the tank destroyer to escape if enemy armour got too close.
The T21 was initially standardised as the M4 37mm gun motor carriage, but there was already an M4 37mm gun carriage for the towed M3 37mm gun, and so the GMC was redesignated as the M6 37mm gun motor carriage.
The M6 was a very simple conversion. The 3/4 ton weapon carrier was a small truck, often used with a simple roof for the driver and co-driver and a cover over the small cargo compartment. On the M6 these were removed, and the vehicle resembled a larger than normal jeep. The gun was simply mounted in the carrying compartment. The test versions had the standard M3 gun shield, but production vehicles included a larger armoured shield to protect the gun crew. Attempts to mount an anti-tank gun on a jeep had failed because of a lack of space, but the M6 was still a rather cramped vehicle.
They entered production with the Fargo Division of Chrysler Corp and a total of 5,380 were built between April and October 1942. The M6 was the cheapest US tank destroyer of the Second World War, costing $4,265 each (compared to $47,905 for the M10 3in Gun Motor Carriage, which was based on the M4 Sherman medium tank).
By the time the M6 GMC was in production the 37mm gun was obsolete. The M3 75mm Gun Motor Carriage was already in production and in June 1942 the M10 3in Gun Motor Carriage was standardized.
In 1941 the US Army considered forming three types of tank destroyer battalions. Heavy battalions would use the M3 75mm Gun Motor Carriage. Light battalions would use the M6 GMC. Light towed battalions would use the M3 towed 37mm gun. The light battalions were soon abandoned and replaced with mixed self-propelled battalions, which were equipped with a mix of the M3 and the M6.
The first tank destroyer battalion to see combat was the 601st Tank Destroyer Battalion, one of the mixed battalions. The 601st took part in Operation Torch, the invasion of North Africa of November 1942. The M3 performed quite well, but the M6 was a total failure. Its 37mm gun was unable to penetrate the armour of the German tanks of late 1942 and early 1943 while its thin armour left it vulnerable to every type of enemy fire.
After this disastrous baptism of fire the surviving M6 GMCs were withdrawn from the tank destroyer battalions and were converted back into normal unarmed weapons carriers (i.e. back into trucks). The poor performance of the M6 encouraged General Patton and other opponents of the tank destroyer concept. They believed that the thinly armoured tank destroyers were too inflexible for use on the fast flowing battlefield, and should be replaced with standard tanks. General McNair's Army Ground Force, which was a major supporter of the tank destroyer, rather played into their hands by arguing that the problem with the M3 and M6 was that they had been used in the wrong roles - as assault guns, on cordon defence or to support infantry - rather than for concentrated counterattacks against German armour.
A few M6s did survive. Some were used in the Pacific during 1943 while others went to the Free French, who used them in Europe in 1944. Most of these French M6s were allocated to units in North Africa in 1943, and were used during Operation Dragoon, the invasion of the south of France in the summer of 1944.
The Marine Corps used a number of M6s in the Pacific. Some took part in the fighting on Bougainville and were still in use early in 1944. They were used with the Special Weapons Companies of the Marine Divisions rather than in specific tank destroyer units.