The M12 had a 400 hp, gasoline powered, Continental R975-C1 engine that could propel the M12 at speeds of up to 38.6 km/h (24 mph). It had a vertical volute suspension system and 5 speed forward, 1 speed reverse transmission.
It also had a crew of six and armor protection varying between 10 mm to 25 mm. The armament of the M12 was an M1918M1 155 mm main gun, a weapon based on an old French design, and a 12.7 mm Browning M2HB. The M12 could also carry ten rounds for its main gun and about 1,000 rounds for the machine gun.
At first the M12 was based on an M3 Lee Medium Tank chassis, but later on about seventy-four models were converted to the M4A3 Sherman chassis. One of the notable features of the M12 was its spade that lowered when it was in the firing position and was raised when moving. This spade was meant to stabilize the main gun when it was firing.
While on the typical Lee or Sherman tank the engine was located in the rear, on the M12 the engine was located in the centre. This was to make room for the main gun and its ammo. Just like the original tanks, the driver and co-driver were in the front of the tank. The gun crew was positioned in rear, to provide easy access to the gun.
The early M3-based M12s were designed for indirect fire. But due to the D-Day invasion plans, which would require the M12 in a direct fire role, the newer M12s were given the Sherman tank's stronger suspension and a gun shield to protect the previously exposed crew. 
The M30 Cargo Carrier was the only variant of the M12 Motor Gun Carriage and it was, as the name suggests, meant to carry cargo. Unlike the M12, the M30 Cargo Carrier had no 155 mm main gun. It had only a 12.7 mm Browning M2HB for defense, with the rest of the empty space used for storage. It was much lighter with a weight of 21,318.8 kg. The length of the M30 was 6 meters and the engine, along with the suspension, were the same as the M12. M30s accompanied artillery formations and carried extra ammo.
The M12 Motor Gun Carriage's first prototype was the T6 and which began testing in 1942. The US Army did not initially approve the M12 despite the fact that towed artillery could take over six times longer to set up and set up again, time that was desperately needed on the battlefield. The M12 began production in March of 1943 with its main manufacturer being the Pressed Steel Car Company. Later on in 1944, 74 of the 100 ordered were refitted on the M4A3 Sherman chassis and they were sent to Europe. They were used in the Battle of the Bulge, Italy, and Germany. Most notably, in the campaigns of 1945, to breach the heavier German defenses that increasingly consisted of thick, concrete bunkers, M12s were often used in the direct-fire role. The M12's 150 mm HE shell was well capable of destroying just about any fortification it came into contact with, earning the vehicle the nickname of "Door-knocker".
- ↑ http://www.battletanks.com/m12_gun_motor_carriage.htm
- ↑ 2.0 2.1 http://www.historyofwar.org/articles/weapons_M12_155mm_GMC.html
- ↑ http://www.wwiivehicles.com/usa/self-propelled-guns/m12.asp
- ↑ Rottman, Gordon L. Victory 1945: Western Allied Troops in Northwest Europe. Osprey Publishing (2015), Page 30