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M3 75mm Self-Propelled Gun

During the early months of World War II, the United States was forced to improvise to provide its forces with self-propelled artillery. An initlal attempt at a self-propelled gun, in June 1940, saw US engineers adapt the venerable 75mm M1897A gun to the M3 halftrack chassis. Adopted in 1941 and obsolete in 1944, the 75mm Gun Motor Carriage M3 had a mere 1,933-yard range, firing up to 15-pound projectiles. US forces used the M3 in all theaters of the war, and, although sometimes used as a tank destroyer, it was more effective as a mobile infantry support weapon. But the 75mm gun was found deficient against modern armor.

Because of the inability to tow 75-mm. guns behind a motor vehicle, Chief of Field Artillery Major General Harry G. Bishop (1930-1934), took aggressive action to find a solution shortly after taking office in 1930. Bishop openly criticized the War Department's reluctance to adopt motorized guns and howitzers for the division. In September 1931 Bishop pointed out that those units that had violated the directive of 1922 to eliminate motor-drawn light artillery were having success with it. At the end of a letter to the Adjutant General on I September 1931, Bishop wrote, "Long continuous study, experimentation and tests have convinced this office [Office of the Chief of Field Artillery] that the prime mover problems can be solved by the use of Ford vehicles (or their equivalent)." 48 Even though rapid improvements in motor transportation and the lack of funds prevented his office from reaching definitive conclusions about the best motor vehicle for division artillery, Bishop still found motordrawn light field pieces to be practical.

Bishop correctly evaluated the advancements in motor vehicles. During the twenties and early thirties, General Motors, International Harvester Company, Marmon-Herrington Company, Ford Motor Company, and other companies were building four- and six wheel trucks with crosscountry capabilities, while the Cleveland Tractor Company, Allis Chalmers Company, the Caterpillar Tractor Company, and Holt Tractor Company were producing dependable and sturdy track tractors with cross-country abilities. Because of these trucks and tractors, Bishop simply could not understand why the War Department cautiously approached motorizing division artillery. As far as the General was concerned in 1931, only the scarcity of money stood in the way of motorizing all of the division's artillery. Because of the decline in the horse population in the United States and a modernization program initiated in 1933 by Chief of Staff, General Douglas MacAtthur (1930-1935), the War Department decided to motorize fifty percent of its light field artillery.

Although combat in Europe encouraged the field artillery to accept self-propelled and organic air observation, many field artillery officers still opposed adopting antitank and antiaircraft missions. In 1942 the field artillery continued to view supporting the infantry and furnishing counterbattery fire as its primary missions. Despite pressure to accept prominent roles in antitank and antiaircraft work, the field artillery successfully resisted adding these missions because the War Department organized tank destroyer units of towed and self-propelled guns and because the coast artillery retained responsibility for antiaircraft artillery.

The 75-mm Gun Motor Carriages M3 and M3A1 mount the 75-mm Gun M1897A4 with Recoil Mechanism M1897A7 and 75-mm Gun Mounts M3 and M5. The 75-mm Gun Motor Carriages M3 and M3A1 are employed chiefly as tank destroyers and the gun is used in direct fire only. Gun is mounted in the approximate center of the vehicle and fires forward. An armor plate gun shield is mounted on the gun carriages. This supplements the protection given to gun and crew by the armor plate windshield and door shields, and by the armored hull.

The original French 75-mm Gun M1897 has had many modifications over the years. However, aside from refinements in barrel and recoil mechanism, and modernization of the carriage, the materiel is basically unchanged in design and effectiveness. The 75-mm Gun M1897A4 with Recoil Mechanism M1897A7 and 75-mm Gun Mount M3 are used in the 75-mm Gun Motor Carriage M3. The 75-mm Gun Mount M3 is a 75-mm Gun Carriage M2A3 (field piece) modified by the elimination of wheels, axle, and equalizers. The 75-mm Gun M1897A4 with Recoil Mechanism M1897A7 and 75-mm Gun Mount M5 are used in the 75-mm Gun Motor Carriage M3A1. The 75-mm Gun Mount M5 is a 75-mm Gun Carriage M2A2 (field piece) modified by the elimination of wheels, axle, and equalizers.

On the 75-mm Gun Mount M3, elevating and traversing handwheels are provided on the left side of the mount and an elevating handwheel is provided on the right side. On the 75-mm Gun Mount M5, only a traversing handwheel is provided on the left side of the mount and an elevating handwheel on the right side

The gun is of the built-up type. It consists of tube, breech hoop, inner locking hoop, outer locking hoop, jacket, muzzle hoop, and gun slide bearings. The tube is rifled with a uniform right-hand twist, one turn in 25.6 calibers. From rear to front, three supports, the jacket support, intermediate support, and muzzle support, are pinned to the tube and serve to hold the gun slide bearings in alinement. The recoil indicator, for determining length of recoil, is assembled to the intermediate support of the right side. A fourth support, the breech support, is pinned to the breech hoop in front of the recoil lug.

c. The breech hoop extends to the rear of tube. The breechblock threads into the breech hoop. The breech hoop also con­ tains pockets for the arms of the extractor and the extractor spindle on which the extractor pivots. A breechblock stop (fig. 15) is hinged between two projections on the left of the breech hoop, limiting rotation of breechblock in the open position. A projection on the lower right of the breech hoop stops the breechblock in the closed position. Just above this projection the breechblock latch catch is inserted and held in place by three headless screws.

The rear sight is fitted into a dovetailed seat in the rear face of the breech hoop at top center and is secured by a blind screw. Although the sight is no longer used, the rim of this sight still acts as a cam to trip the breechblock latch when the breechblock is being opened. This frees the breechblock catch so that it can en­ gage the breechblock latch catch when the breechblock is closed.