M116 howitzer

The 75 mm pack howitzer was designed in the United States in the 1920s to meet a need for an artillery piece that could be moved across difficult terrain. Development began in 1920, and in August 1927, the weapon was standardized as Howitzer, Pack, 75mm M1 on Carriage M1. Due to meager funding, production rates were very low; by 1933, only 32 guns had been manufactured, and by 1940, only 91 pieces. It was not until September 1940 that the howitzer was put into mass production. By then, the M1 had been succeeded by the slightly modified M1A1. Production continued until December 1944.[2][3]

The only significant changes during the mass production period were carriage improvements. The original carriage M1 was of box trail type, with wooden wheels. The requirement for a lightweight howitzer for airborne troops led to the introduction of the M8 carriage, similar except for new wheels with pneumatic tires.

Another requirement, from the cavalry branch of the US Army, resulted in a completely different family of "field howitzer" split trail carriages M3A1 / M3A2 / M3A3. However, only limited numbers of the M1 field howitzer variant were built, due to cavalry's switch to self-propelled guns.[2][3]

The howitzer M1 or M1A1 consisted of tube and breech, which were joined together by interrupted threads, allowing for quick assembly and disassembly. One eighth of a turn was required to connect or disconnect tube and breech. The tube had uniform, right hand rifling with one turn in 20 calibers. The breech was of horizontal sliding-block type, with continuous-pull firing mechanism.[5]

The recoil system was Hydro-pneumatic. Both recoil buffer and recuperator were located under the barrel.[5]

The pack howitzer carriage M1 had dismantling construction. The carriage was of box trail type, with steel-rimmed wooden wheels.[6] For transportation, the howitzer M1 or M1A1 on carriage M1 could be broken down into six mule loads, with payload weight between 73 kg (161 lb) and 107 kg (236 lb) each:

• Tube
• Breech and wheels
• Top sleigh and cradle
• Bottom sleigh and recoil mechanism
• Front trail
• Rear trail and axle.[5]

The carriage M8 was identical, except for axle arms and wheels, which were metal with pneumatic tires.[6] The howitzer on carriage M8 could be broken down into seven mule loads or into nine parachute loads (the latter arrangement included 18 rounds of ammunition). It could also be towed by vehicle such as 1/4 ton jeep, or transported by plane or glider such as CG-4 Waco.[7]

The field howitzer carriages of the M3 family were non-dismantling, split trail. All these were fitted with metal wheels with pneumatic tires; all had an additional retractable support, referred to as a firing base. In firing position, the firing base could be lowered and wheels raised, leaving the weapon to rest on the firing base and trails.[5]

• 
  75 mm howitzer on carriage M1.
• 
  75 mm howitzer on carriage M8.
• 
  75 mm howitzer on carriage M3A3.
• 
  75 mm howitzer on carriage M3A3.

75 mm Howitzer M1920.

75 mm howitzer on carriage M3A1, fitted with 37 mm subcaliber gun for training.

75 mm pack howitzer on display at the National World War II Museum

• Howitzer variants:
  • M1920, M1922A, M1922B, M1923B, M1923E1, M1923E2 – prototypes.[5]
  • M1 – the first standardized variant. Based on M1923E2 with minor changes.[5]
  • M1A1 – variant with modified breech block and breech ring.[5]
  • M2 – vehicle mounted variant. Tube and breech from M1A1 were used. In order to provide a cylindrical recoil surface, the tube was fitted with an external sleeve. 197 built.[15]
  • M3 – vehicle mounted variant; like M2 but with recoil surface as a part of the tube. The M2 and M3 barrels were interchangeable.[15]
  • M116 – post-war designation of the complete weapon.
  • M120 – post-war designation for saluting howitzers
• Carriage variants:
  • M1 – dismantling box trail carriage with wooden wheels.[6]
  • M2A1, T2, T2E1, T2E2, T2E3 – experimental carriages.[5]
  • M3A1 – split-trail carriage with firing base and pneumatic tires.[6]
  • M3A2 – M3A1 with shield added.[6]
  • M3A3 – M3A2 with different wheels and combat tires.[6]
  • M8 – M1 with new wheels with pneumatic tires.[6]

75 mm Howitzer Motor Carriage T30.

Howitzer Motor Carriage M8.

Two nearly identical vehicle mounted variants – M2 and M3 – were developed based on tube and breech of the M1A1, for use in the 75mm Howitzer Motor Carriage T47 / M8. Both variants had a cylindrical "recoil surface" around the tube. In the M2, the surface was provided by use of a separately manufactured sleeve, while in the M3 it became an integral part of the barrel. M2 and M3 were fully interchangeable.[16] These guns were mounted on the below armored vehicles:

• 75mm Howitzer Motor Carriage T47 / M8 – M2 / M3 in mount M7.[16]
• Landing Vehicle Tracked (A)-4 – M3 in mount M7.[17]
• Landing Vehicle Tracked (A)-5 – M3 in mount M12.[18]

In addition, M1 / M1A1 was mated to a number of other vehicles. Only the T30 reached mass production.

• Medium Tank T5E2 – M1A1.[19]
• Experimental mount on Holt light tractor.[20]
• 75mm Howitzer Motor Carriage T1 (Tank development chassis T2).[20]
• 75mm Howitzer Motor Carriage T3 (Combat Car M1 chassis).[21]
• 75mm Howitzer Motor Carriage T17 (Combat Car M1E3 chassis).[22]
• 75mm Howitzer Motor Carriage T18 (Light Tank M3 chassis) – M1A1.[22]
• 75mm Howitzer Motor Carriage T30 – M1A1 in mount T10.[23]
• 75mm Howitzer Motor Carriage T41 (Light Tank M5 chassis).[24]

The gun fired fixed (HEAT M66) and semi-fixed ammunition, fitted with 75mm Cartridge Case M5A1 (type II) and (type I) accordingly. The propelling charge of the semi-fixed ammunition consisted of base charge and three incremental charges, forming four different charges from 1 (the smallest) to 4 (the largest).[25]

HEAT M66 Shell penetrated about 91 mm (3.6 in) of homogeneous armor at 0 degrees at any range.[26] While the powder charge was different, this gun fired the same types of shell projectiles as used in the 75mm Tank Guns of WWII, which were themselves derived from the 75×350 mm R ammunition of the French 75mm field gun of WWI.

† - The blank likely existed as M337, M337A and M337A1 based on US Army model numbering convention.

• Brazil
• People's Republic of China - captured from Nationalist forces during the Chinese Civil War.
• Croatia - 12 are kept in active service primarily as ceremonial cannons.
•  France[28]
•  Greece[1]
•  Japan
•  Laos - supplied to the Royal Lao Army during the First Indochina War and used during the Laotian Civil War.
•  Pakistan
•  Philippines
• Republic of China - supplied to Nationalist forces during World War 2 and used during the Chinese Civil War.
•  South Korea: A total of 24 M1s were supplied to the Army in August 1950.[29]
•  Turkey[1]
•  United States
•  United Kingdom
• Republic Of Vietnam - ARVN used this howitzer during the Vietnam war. Some of them were captured by Northern Vietnamese Forces.
•  Vietnam: Viet Minh[30] and People's Army of Vietnam[31]
•  Yugoslavia

• Skoda 75 mm Model 1928
• 7.5 cm KwK 37, the closest WW II German vehicular-mount equivalent to the M2 and M3 versions
• List of U.S. Army weapons by supply catalog designation SNL C-20
• Artillery observer

• Hogg, Ian V. (1998). Allied Artillery of World War Two. Crowood Press, Ramsbury. ISBN 1-86126-165-9.
• Hunnicutt, R. P. (1992). Stuart: A History of the American Light Tank. Presidio Press. ISBN 0-89141-462-2.
• Hunnicutt, R. P. (1994). Sherman: A History of the American Medium Tank. Presidio Press. ISBN 0-89141-080-5.
• Hunnicutt, R. P. (2001). Half-Track: A History of American Semi-Tracked Vehicles. Presidio Press. ISBN 0-89141-742-7.
• Zaloga, Steven J. (2007a). US Field Artillery of World War II. New Vanguard 131. illustrated by Brian Delf. Osprey Publishing. ISBN 978-1-84603-061-1.
• Zaloga, Steven J. (2007b). US Airborne Divisions in the ETO 1944–45. Battle Orders 25. Osprey Publishing. ISBN 978-1-84603-118-2.
• Technical Manual TM 9-319, 75mm Howitzer M1A1 and Carriage M8. War Department, 1948.
• Technical Manual TM 9-1320, 75mm Howitzers and Carriages. War Department, 1944.
• Technical Manual TM 9-1901, Artillery Ammunition. War Department, 1944.
• Technical Manual TM 9-2005 volume 3, Infantry and Cavalry Accompanying Weapons. War Department, 1942.
• "History of U.S. Marine Corps Operations in World War II". Retrieved 16 September 2007 – via HyperWar Foundation.
• "The Marine Division". WW2Gyrene.org. Archived from the original on 8 June 2019. Retrieved 16 September 2007.
• Anderson, Rich. "US Army in World War II". Military History Online. Retrieved 16 September 2007.
• "US Guns in UK Service". British Artillery in World War II. 14 June 2014. Archived from the original on 2 February 2016. Retrieved 16 September 2007.

Wikimedia Commons has media related to M116 75 mm howitzer.

• The Old Army Mules Takes Guns Where Wheels Don't Go May 1941 Popular Science
• "Airplane Moves Artillery 120 Miles In An Hour", June 1931, Popular Science air transport of early version of M116 Howitzer