M116 howitzer

The 75mm Pack Howitzer M1 (also known by its post-war designation M116) was designed in the United States in 1920s to meet a need for an artillery piece that could be moved across difficult terrain. The gun and carriage was designed so that it could be broken down into several pieces to be carried by pack animals. The gun saw combat in Second World War with the US Army (primarily used by airborne units), with US Marine Corps, and was also supplied to foreign forces.

In addition to the pack / air portable configuration, the gun was mounted on a conventional carriage to serve as a field artillery piece. Derived vehicle mounted howitzers M2 and M3 were used in the 75mm HMC M8 and some LVT models. In addition, the M1 in its original version was mated to a number of other self-propelled carriages, though only one of those – 75mm HMC T30 – reached mass production.

Development and production
The 75 mm pack howitzer was designed in the United States in 1920s to meet a need for an artillery piece that could be moved across difficult terrain. In August 1927, the weapon was standardized as Howitzer, Pack, 75mm M1 on Carriage M1. Due to meager funding, production rates were low; by 1940, only 91 pieces were manufactured. Only in September 1940, a year into World War II, was the howitzer put into mass production. By then, M1 was succeeded by the slightly modified M1A1. The production continued until December 1944.

The only significant changes during the mass production period were carriage improvements. The original carriage M1 was of box trail type, with wooden wheels. Requirement for a lightweight howitzer for airborne troops led to introduction of the M8 carriage, similar except 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 number of the M1 in field howitzer variant were built, due to cavalry's switch to self-propelled guns.

Description
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.

The recoil system was hydropneumatic. Both recoil buffer and recuperator were located under the barrel.

The pack howitzer carriage M1 had dismantling construction. The carriage was of box trail type, with steel-rimmed wooden wheels. For transportation, the howitzer M1 or M1A1 on carriage M1 could be broken down into six mule loads, with payload weight between 73 and 107 kg each:
 * Tube
 * Breech and wheels
 * Top sleigh and cradle
 * Bottom sleigh and recoil mechanism
 * Front trail
 * Rear trail and axle.

The carriage M8 was identical, except for axle arms and wheels, which were metal with pneumatic tires. 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.

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.

US forces
In the Second World War era US Army, 75 mm howitzers were issued to airborne and mountain units.

An airborne division, according to the organization of February 1944, had three 75 mm howitzer battalions – two glider field artillery battalions (two six-gun batteries each) and one parachute field artillery battalion (three four-gun batteries), in total 36 pieces per division. In December 1944, new Tables of Organization and Equipment increased the divisional firepower to 60 75 mm howitzers (as an option, in glider battalions 75 mm pieces could be replaced with more powerful 105mm M3).

The only mountain division formed, the 10th, had three 75 mm howitzer battalions, 12 pieces each. The gun was also used by some separate field artillery battalions.

In the US Marine Corps, under the E-series Tables of Organization (TO) from 15 April 1943 divisional artillery included three 75 mm howitzer battalions, 12 pieces each. The F-series TO from 5 May 1944 reduced the number of 75 mm battalions to two, and the G-series TO removed them altogether, completing the shift to 105 mm and 155 mm howitzers. Although the G-series TO was only adopted on 4 September 1945, in practice in some divisions the change was introduced early in 1945.

As of 2012, the M116 was still used by the US Army for ceremonial purposes.

Other operators
Two major lend lease recipients of the M1 were United Kingdom (826 pack howitzers) and China (637 pack howitzers and 125 field howitzers). 68 pieces were supplied to France, and 60 to various countries in Latin America.

In the British service, the howitzer was issued to two mountain artillery regiments, two airlanding light artillery regiments, raiding support regiment and was temporary used by some other units. The gun remained in British service until late 1950s.

The 75 mm howitzer was also used by Australian forces – two mountain batteries and some other units.

A single howitzer was airdropped in April 1945 to the 2nd (Italian) SAS Regiment, a special force composed by partisans with mixed political allegiance, Russian ex-POWs, and Wehrmacht deserters, coordinated by Major Roy Farran. Baptized "Molto Stanco" ("Very tired" in Italian), the gun was used in the course of Operation Tombola to harass enemy convoys driving up and down along Route 12 between Modena and Florence. On 21 April 1945 the howitzer was towed by means of a Willys Jeep to the outskirts of Reggio Emilia, from where the Italian gunners initiated a shelling of the city that wrought panic among Axis troops. Believing that the arrival of Allied forces was imminent, the Germans and their fascist allies evacuated the city.

The experience of the guns given to the army of the Republic of China was particularly notable. After the loss of the mainland, the guns left behind were used both by the People's Liberation Army and exported to the Viet Minh. There is record of these guns being used at the Siege of Dien Bien Phu and, presumably, in later conflicts. The type by virtue of its compactness also remained in use with the ROCA's outlying island garrisons as coastal artillery.

153 M116s were supplied to Japan (Japan Ground Self-Defense Force) and they were used until 1980s.

As of 2010, the M116 75 mm pack howitzer is still used by the Turkish Army in operations against Kurdish separatists in southeastern Turkey.

The howitzers have been used for avalanche control at several western US ski areas.

Variants

 * Howitzer variants:
 * M1920, M1922A, M1922B, M1923B, M1923E1, M1923E2 – prototypes.
 * M1 – the first standardized variant. Based on M1923E2 with minor changes.
 * M1A1 – variant with modified breech block and breech ring.
 * 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.
 * M3 – vehicle mounted variant; like M2 but with recoil surface as a part of the tube. The M2 and M3 barrels were interchangeable.
 * 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.
 * M2A1, T2, T2E1, T2E2, T2E3 – experimental carriages.
 * M3A1 – split-trail carriage with firing base and pneumatic tires.
 * M3A2 – M3A1 with shield added.
 * M3A3 – M3A2 with different wheels and combat tires.
 * M8 – M1 with new wheels with pneumatic tires.

Self-propelled mounts
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 cylindlical "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. These guns were mounted on the below armored vehicles:


 * 75mm Howitzer Motor Carriage T47 / M8 – M2 / M3 in mount M7.
 * LVT(A)-4 – M3 in mount M7.
 * LVT(A)-5 – M3 in mount M12.

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


 * Medium Tank T5E2 – M1A1.
 * Experimental mount on Holt light tractor.
 * 75mm Howitzer Motor Carriage T1 (Tank development chassis T2).
 * 75mm Howitzer Motor Carriage T3 (Combat Car M1 chassis).
 * 75mm Howitzer Motor Carriage T17 (Combat Car M1E3 chassis).
 * 75mm Howitzer Motor Carriage T18 (Light Tank M3 chassis) – M1A1.
 * 75mm Howitzer Motor Carriage T30 – M1A1 in mount T10.
 * 75mm Howitzer Motor Carriage T41 (Light Tank M5 chassis).

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

HEAT M66 Shell penetrated about 91 mm of homogeneous armor at 0 degrees at any range.