Key to the design of the new SPG gun platform was the selection of the M1917 series 155mm field gun. The M1917 was a proven commodity and the US Army held stocks of the weapon from its days participating in World War 1. The M1917 was essentially the French 155mm GPF which gave excellent service during its heyday in the Great War and it made logistical sense once again to couple an existing weapon with an existing chassis. The gun was available in the "M1917" and "M1917A1" flavors and also appeared in an "M1918 M1" form. Depending on existing gun stocks of these gun barrels, the new SPG vehicle could make use of any three as available - all firing a large 155mm high-explosive shell. The recoil mounting mechanism supported the weapon atop the chassis just aft of the vehicle's center. While traversal was limited, elevation was possible to an extent. Born as an indirect-fire field gun (and not a direct-fire weapon such as an anti-tank gun), the M1917/M1918 series guns were proven weapons at lobbing explosive devices against target areas. Self-defense was through a single .50 caliber Browning M2 heavy machine gun - suitable for engaging light-armored vehicles, infantry and low-flying aircraft. Since such SPG platforms would be operating behind the main line of advance, this defense was deemed sufficient. Additionally, any personal weapons carried by the crew could be brought to bear. The new SPG vehicle was designated in the US Army inventory as the "M12".
The M12 weighed in at 26 tons and was powered by a Continental R975 EC2 gasoline engine as in the M3 Grant/Lee series tanks. Maximum speed was approximately 23 miles per hour on ideal surfaces with an operational range of 135 miles though performance dropped off considerably when off-road. The chassis sat atop a Vertical Volute Suspension System (VVSS) common to American tracked vehicles of the period and this automotive arrangement allowed the vehicle to keep pace with other mechanized army forces as needed.
In service, M12 production was extremely limited for more capable SPG solutions arrived in time. As such, approximately 100 vehicles were only ever produced and many were initially utilized for training of new gunnery, driver and command crews in the fine art of battlefield management while other systems went unused in storage. However, for the mid-1944 D-Day landings in Northern France, some 74 M12 vehicles were prepared for action with additional equipment and these systems ultimately found their way into Europe with the advancing US Army on their march across France. The M12 proved its worth in engaging specific target areas with their 155mm ordnance while also being called upon to lay waste to complete structures occupied by the enemy. By this time, the "M30 Cargo Carrier" - a similar M3 Grant/Lee-minded conversion though lacking the 155mm artillery gun - was fielded alongside M12 gun carriers as dedicated ammunition carriers for the M12 hull was limited to carrying just 10 x 155mm projectiles aboard. The M30 ammunition carrier added a further 40 x 155mm projectiles to each pair of M12 gun systems fielded.
Only a single M12 example survives today (2013), this under the care of Fort Sill in the United States. The M12 was given the formal US Army designation of "155mm Gun Motor Carriage" in keeping with US Army military nomenclature of the time. Similarly, the M30 carrier was known as the "Cargo Carrier M30".
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