JR Potts, AUS 173d AB (Updated: 9/9/2016):
The 240mm, 9.5-inch M1918 Schneider howitzer of French origins was the largest-caliber field gun to be deployed by US Army field artillery in World War 1 and was used to great success. The gun began life as a towed artillery piece originally designed during World War 1 and since having been updated on many occasions in increase potency and keep the type modernized for current battlefield requirements. The 9.5-inch barrel fired a hefty 356-pound shell which was propelled by a 49-pound bag of TNT and could reach out to 14,000 yards - approximately 10 miles - when fired.
The M1918 gun was towed in separate major components by no fewer than six tractors and required a full 20-man crew to transport, setup and operate. In a combat situation, the crew would have to set up the gun system in the cover of darkness, which also required reconnaissance of the ground and much training as deployment of the six tractors and the many required hours needed to set up the gun in broad day light would have drawn much attention from enemy artillery. When the precise firing position was found, a seven-man lead crew was first on the scene to layout the emplacement pit using a surveyor's transit. The crew then dug a 7x6x3 foot pit with conventional picks and shovels over the course of three hours if the ground proved fair. An additional twelve hours and the balance of the 20-man crew was needed to complete setup of the gun.
Firstly, a metal base was dropped into the prepared pit with steel ramps placed along the sides of the pit to help distribute the weight of the system. Next came the tractor towing the aiming mounts followed by another tractor bringing the recoil mechanism and, finally, another transport hauling the 11,000 pound barrel. The guns wheels alone weighed up to 1,000lbs each. When completed, the entire gun system weighed an impressive 39,920lbs. As one can see, the massive effort made a self-propelled version very desirable.
The Caterpillar Mark IV, developed in 1920, did not make use of a "true" tank chassis but instead relied upon a tractor chassis - weighing some 31,580 pounds - with an 11,000lb barrel and applicable firing system added. The tractor was based on a French design produced by the St. Chamond Company. All told, the completed Caterpillar vehicle topped 71,500 pounds. A 150 horsepower motor powered a 400 volt, 70kW generator which, itself, powered the 70 horsepower electric motors required to move the track systems. A crane, connected to the back of the gunnery platform, assisted in raising each 356-pound shell from awaiting ammunition carts. This arm then swung the projectiles into place on the loading tray. The primary tractor could transport seven crew. Testing ensued at the Aberdeen Proving Grounds of Maryland in 1921.
As a mover, the tractor chassis proved underpowered with its engine/motor arrangement for a top speed of just 9 miles per hour was possible and this only in ideal conditions. A second tractor (sans the gun and mounting), known as the "Mark IV-A", was attached to the rear of the primary mover in an attempt to help increase speeds over open ground. While towing the vehicle itself would have been an option, it would defeat the purpose of its "self-propelled" nature. Thusly, the Mk IV concept was ultimately deemed inadequate and later scrapped from US Army requirements, this after six of the kind were completed.
The heavy-caliber self-propelled gun concept was tried once again during World War 2 when, in 1944, a 240mm howitzer was mounted onto a heavy tank T26E3 tracked chassis. An extra wheel bogie had to be fitted to each track side due to the excessive weight. This new variant became the "T92 Howitzer Motor Carriage" though the war was over by September of 1945. The T92 HMC was never used in combat and later scrapped - along with the dream of a US Army large-caliber self-propelled gun. Similar gun concepts within Germany and the Soviet Union also fell to naught during the war.