When the U.S. Army and later, the U.S. Marines, arrived in Europe to take part in World War One, they did so without their own artillery. This was due to the U.S. M1902 light field gun firing a 3-inch (76.2mm) shell and so the U.S. Army elected to leave their M1902 guns behind and adopt the French Canon de 75 Modèle 1897 as the M1897 75mm gun. Because of this, the Marines also left their artillery back in the United States and so while in Europe, they were mostly dependent on the U.S. Army for artillery support. So, while the U.S. Army also adopted the French heavy Canon de 155 C Modèle 1917 Schneider as the M1917 155mm gun, the Marines had nothing comparable. This prompted the Marines to consider a heavy gun of their own. Back in the United States, the Marines found that the U.S. Navy had a surplus of 7"/45 caliber guns Mark 2. These guns had been developed back in 1900 and were deployed onboard Connecticut-class and Mississippi-class pre-dreadnought battleships as secondary guns beginning in 1906. However, prior to World War One, most of these guns had been removed from Navy warships. So, early in 1918, the Marines designed a wheeled carriage to mount the guns on but the result was completely unsuited for the battlefield due to its immense 32 ton weight and 6.6 foot diameter wheels. Such a carriage would have sunk into the mire that was seemingly ever present in France. Returning to the drawing board, the Marines went to the U.S. Navy to seek their assistance and what the naval engineers at the Bureau of Ordnance came up with was to mount two slightly modified Holt tractor track assemblies to the gun carriage. The track assemblies were already, more or less, in production by the Holt Manufacturing Company which sped up development and eased construction. In addition, the tracks spread the weight of the gun across more area and thus lightened the ground pressure. By consequence, cross-country mobility was improved. Another benefit was the gun did not need any special preparation to fire. This same concept was seen again in World War Two with the Russian 203mm M1931 howitzer.
On May 15, 1918, the 7” Tracked Gun Mk. 2 prototype was completed by the Baldwin Locomotive Works in Philadelphia, Pennsylvania and soon after, the Marines ordered 20 of the guns and the U.S. Army ordered 36 examples. By the early winter of 1918, the majority of the order for the Marines had been completed and 18 guns, along with Holt tractors to pull the guns, were assigned to the 10th. Marine Artillery Regiment. The regiment took the guns to Lower Station, located in Dahlgen, Virginia and on October 16, 1918, the first shots were fired from the guns. World War One, however, ended on November 11, 1918 and so the Marines canceled the remaining two guns and the U.S. Army saw 20 guns delivered before the rest of the purchase was cancelled as the need for the guns had past. In time, the Marines began to fill out their artillery forces with the M1897 and M1917 guns and so the 7” Tracked Guns Mk. 2 of both the Marines and the Army were retired from service and put into reserve. The advent of World War Two saw the guns taken out of mothballs and returned to duty as coastal defense guns with the majority under the control of the U.S. Army Coast Artillery Corps. Most of the guns were deployed to Hawaii with 16 guns positioned around the islands with 2 guns located at Fort Rosecrans in San Diego, California. Another 8 guns were situated at Bora Bora, French Polynesia and 4 guns were part of the inventory of the U.S. Marine 6th. Defense Battalion on Midway Island. The end of World War Two also spelled the end of the road for the 7”/45 caliber gun.
The performance of the 7”/45 (178mm) caliber gun was impressive. A good crew could fire four 152lb. armor-piercing shells every minute. The carriage permitted a maximum elevation of 40 degrees and at that elevation, the muzzle velocity of 2,700 feet per second gave the gun a range of 13.7 miles. In 1920, the U.S. Army stated it was the most powerful gun it had ever taken into service. By comparison, the M1917 was only capable of a range of 7 miles firing a 100lb. shell. Today, only a single example of the 7” Tracked Gun Mk. 2 exists today and it is located at the Naval Surface Warfare Center Dahlgren in Dahlgren, Virginia.
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