155 mm gun M1

A wooden mock-up of a modified GPF gun with equilibrators to increase maximal elevation and range, 1920

Before entering World War I, the United States was poorly equipped with heavy artillery. To address this problem a number of foreign heavy artillery guns were adopted, including the Canon de 155 mm GPF. After the end of the war the Westervelt Board was convened to assess the artillery experience of the combatant powers and map out future directions for the US Army artillery. The conclusion of the board vis-a-vis heavy field artillery was that the French 155 mm GPF should be adopted as the standard heavy field piece but further development work should occur to achieve a heavy field gun with a max. range of 25,000 yards (23 km), a vertical arc of fire from 0° to 65° (for comparison, GPF had only 35°), a projectile not exceeding 100 lb (45 kg) and the capability to be installed on a mount with either caterpillar tracks or rubber tires.[2] A number of prototypes were produced in the 1920s (M1920 and M1920M1 were even standardized) and 1930s (a new design was started from scratch after 1929), the projects were repeatedly put on hold due to lack of funds.[3] Developed in the summer of 1930, the radical split-trail carriage for both 155-mm gun and 8-inch howitzer designated T2 was the first in the US to feature an all-welded construction as well as a 8-wheel 2-axis roll-bearing bogie for high-speed mobility.[4] In 1938 the 155 mm gun T4 on carriage T2 was finally adopted as 155 mm gun M1 on carriage M1.[5]

Long Tom at crew training in England

The Long Tom saw combat for the first time in the North African Campaign on December 24, 1942, with "A" Battery of the 36th Field Artillery Regiment. Eventually it equipped 33 U.S. Army battalions in the European and Mediterranean Theaters (the 173rd, 190th, 200th, 208th, 240th, 261st, 273rd, 514th–516th, 528th, 530th, 540th, 541st, 546th–549th, 559th, 561st, 634th, 635th, 731st, 733rd, 734th, 766th, 976th–981st, 985th and 989th), and 8 in the Pacific Theater (the 168th, 223rd, 226th, 433rd, 517th, 531st, 532nd, and 983rd). The 353rd, 732nd, and 993rd Field Artillery Battalions were segregated 155 mm gun units that never went overseas. The 353rd was converted to the 1697th Engineer Combat Battalion (Colored) on 19 March 1944 at Camp Van Dorn, Mississippi, the 732nd was converted to the 1695th Engineer Combat Battalion (Colored) on 15 March 1944 at Camp Pickett, Virginia, and the 993rd was converted to the 1696th Engineer Combat Battalion (Colored) on 19 March 1944 at Camp Swift, Texas.[15]

The 155 mm gun was also used by several Marine defense battalions, notably during Operation Cartwheel in 1943.

The preferable prime mover was initially the Mack NO 6×6 7½ ton truck; from 1943 on, it was supplemented by the tracked M4 High Speed Tractor.[16] 72 rounds of ammunition plus propelling charges could be carried in the M21 4-ton, 2-wheel ammunition trailer; 16 rounds of ammunition plus propelling charges could be carried in the M10 1-ton, 2-wheel ammunition trailer that was often used because of shortages of the former. The later heavy M23 8-ton, 4-wheel ammunition trailer introduced in 1945 could carry 96 rounds of ammunition plus propelling charges.

A small number of Long Tom guns were authorised for supply via Lend-Lease channels, to the United Kingdom (184) and France (25).[17] The authorised establishment of British batteries (excluding training units), including four batteries from the Dominion of Newfoundland, totalled 88 guns.

M2 during the Battle of Okinawa.

M40 in the US Army Ordnance Museum.

Gun variants:

• M1920 – prototype.
• T4 – prototype.
• M1 (1938) – first production variant, 20 built.
• M1A1 (1941) – modified breech ring.
  • M1A1E1 – prototype with chromium-plated bore.
  • M1A1E3 – prototype with liquid cooling.
• M2 Standard (1945) – with modified breech ring.

Carriage variants:

• T2 – prototype.
• M1 (1938).
• M1A1 – refurbished T2 carriages.
• M2 Standard

Limber variants:

• M1 Standard (1938)
• M5 Heavy (1945)

The gun was also mounted on a modified M4 medium tank chassis, in mount M13. The resulting vehicle was initially designated 155 mm Gun Motor Carriage T83 and eventually standardized as 155 mm Gun Motor Carriage M40.[18] 155 mm Gun Motor Carriage T79, based on T23 Medium Tank chassis, never advanced past proposal stage.[19] A portable "Panama mount" M1 was also provided.

British gunners cleaning shells, Italy, February 1945

The gun utilized separate loading, bagged charge ammunition. The propelling charge consisted of base (9.23 kg) and increment (4.69 kg). The data in the table below is for supercharge (base and increment).

• List of U.S. Army weapons by supply catalog designation SNL D-24
• 15 cm Kanone 18 German equivalent
• 152 mm gun M1935 (Br-2) Soviet equivalent, built only in small numbers
• Cannone da 149/40 modello 35 Italian equivalent, only few built

• Bishop, Chris (1998). Encyclopedia of Weapons of World War II. ISBN 0-7607-1022-8.
• Hunnicutt, R. P. (1994). Sherman: A History of the American Medium Tank. Presidio Press. ISBN 0-89141-080-5.
• Hunnicutt, R. P. (1996). Pershing, A History of the Medium Tank T20 Series. Feist Publications. ISBN 1-112-95450-3.
• Williford, Glen M (2016). American Breechloading Mobile Artillery 1875–1953. Thomas D. Batha. Schiffer Publishing Ltd. ISBN 978-0-7643-5049-8.
• Zaloga, Steven J. (2007). US Field Artillery of World War II. New Vanguard 131. illustrated by Brian Delf. Osprey Publishing. ISBN 978-1-84603-061-1.
• Hunnicutt, R. P. (1996). Pershing, A History of the Medium Tank T20 Series. Feist Publications. ISBN 1-112-95450-3.
• Technical Manual TM 9–350, 155-MM Gun M2; Carriage Ml AND M1A1; Gun Mount M13; Heavy Carriage Limber M2 AND M5; AND Firing PlatformM Ml. War Department, 1944.
• Technical Manual TM 9-1901, Artillery Ammunition. War Department, 1944.
• Technical Manual TM 9-1904, Ammunition Inspection Guide. War Department, 1944.
• Technical Manual TM 9-3305, Principles of Artillery Weapons. Department of the Army, May 1981.
• Field Manual FM 6–40, Field Artillery Gunnery. War Department, June 1945.

Wikimedia Commons has media related to M1 Long Tom 155 mm gun.

• 155-mm. Novel Mechanical Features Give Big-Gun Power, Small Gun Speed, November 1942, Popular Science early article with many photos and detailed drawings showing how it is brought into firing position from travel mode.