The Chief of Ordnance ordered a pilot model to have a 155 mm M1918M1 gun put onto a M3 medium chassis.1,5 This was designated the T6 and work began on it in June 1941.1,5
The engine was located in the front of the M12.
The vehicle commander and driver sat in a compartment at the front of the M12.6
The driver and assistant driver had direct and indirect vision. In the M12 there were seats for 4 other crewmen. There was also a hinged platform for the crew for firing.
The M12 provided armor protection for the crew.
The 155 mm M1918 was formerly a French gun that was used as towed artillery in World War I.6 After World War I they were placed into storage.6
The 155 mm gun fired a 95 lb shell at a muzzle velocity of 2,380'/sec and a range of 18,000 yards.4
The M12 had a spade at the rear to help stabilize against the recoil of the gun.1 This could be raised when moving.
The T6 prototype was manufactured by Rock Island Arsenal. Pressed Steel Car Company fitted French M1917 or US made M1918 guns.1
The T6 prototype started testing at the Aberdeen Proving Ground in February 1942.1,5 It had some modification done and was sent to Fort Bragg for the Field Artillery Board to test.5 The T6 proved acceptable, except to the Army Ground Forces, who felt towed artillery was good enough.1,6
During tests the T6 was fired, moved six miles, and then fired again.5 This took a total of 35 minutes to complete.5 A tractor towed 155 mm gun required three hours to do the same thing.5 The Ordnance Board ordered 50 initially, which was increased to 100.1,5
Initial production of 100 of the M12 and M30 were completed in March 1943 by Pressed Steel Car Company.5 These were stored or used for training.5
When the Normandy landings was getting closer it was decided to remanufacture them.5 Baldwin Locomotive Works rebuilt 74 of them by May 1944 with the M4 chassis.1,5
M12 Gun Motor Carriage:
Manufactured by: Pressed Steel Car Company4,5
M30 Cargo Carrier:
Manufactured by: Pressed Steel Car Company5
Production: late 1943 - March 19446
M12 Gun Motor Carriage:
M30 Cargo Carrier: Identical to M12 except no gun.6 There was on M30 for each M12.6 The crew primarily traveled in the M30.6 There was a tailgate to help unload the ammunition. A ring mount for a .50 cal MG was installed in the rear. There was a canvas cover over the crew area.6
Eventually 74 were sent to Europe in June 1944.5,6 They were used in the taking of Cologne5 and in busting the bunkers in the Siegfried Line1.