155mm Field Howitzer M117

The 155mm Field Howitzer M117 is the quintessential field artillery piece of the modern American military. A comprehensive evolution of the older 1940s era M114 Howitzer, the M117 has been improved and modified in almost every way. It was borne out of an American attempt in the 1950s to create a 155mm Howitzer that could be air-dropped in support of paratroopers and, although this project failed, the results were re-directed as a comprehensive upgrade kit of the existing M114 to create a lighter, more effective weapon system. This kit has been widely exported to allies of the United States which also use the M114, such as Japan, Sweden and Switzerland, although the newer Howitzer is now produced in large quantities by American factories.

Design


The M117 is very different from its predecessor, although the basic weapon is still there. The gun itself is essentially the same, but it has been fitted with a new breech mechanism: replacing the interrupted screw breech with a new, faster-firing, semi-automatic horizontal sliding block similar to that of the 75mm Regimental Gun M121. This requires fixed brass cartridge ammunition - which is automatically extracted by a claw as the breech self-opens after firing - and the separate-loading, bagged charges of the M114 are not compatible. The M117 also fires shells which are thicker and can withstand greater blast and pressure but are also quite a bit longer, allowing higher muzzle velocities while still slightly increasing their overall payload. The second major modification is a new, lengthened, light-weight 36-calibre barrel (as opposed to the M114's 23-calibre barrel) with an efficient triple-baffle box-type muzzle brake. Further alterations include a new gunshield with angled "wings" like the 75mm Regimental Gun M121, a broadly similar but lighter-weight carriage and a stronger, lighter recoil mechanism.

Versions
The time taken to apply the M117 modifications to existing M114s was substantial and not always done simultaneously. As such, although existing howitzers in the U.S. Army have been fully upgraded, there are several "interim" designations for partially modified M114s.

155mm Field Howitzer M117: Parent / test model, 16 built.


 * 155mm Field Howitzer M117A1: M114 retrofitted with M117 kit breech.


 * 155mm Field Howitzer M117A2: M114 retrofitted with M117 barrel


 * 155mm Field Howitzer M117A3: M114 retrofitted with new barrel and breech


 * 155mm Field Howitzer M117A3B1: New barrel, breech and recoil mechanism

155mm Field Howitzer M117A3B2: Definitive / production model with all modifications, identical to the M117 parent model and usually just called "155mm Field Howitzer M117". All U.S. Army and Marine Corps M114s have been upgraded to this standard.

155mm Howitzer M118: an M117 modified for use in self-propelled mounts. Main armament of M50 self-propelled howitzer, a derivative of the M48 Pershing tank.

155mm Howitzer M118A1: Short-barrel version of M118 self-propelled mount. Main armament of the M65A2 Assault Howitzer ( 155mm Gun Motor Carriage, Tracked, M65A2) variant of the M65 Assault Gun.

Ammunition
As stated above, the M117 uses single-piece ammunition. The shell and its propellent cartridge - in the form of a long brass casing - are conjoined as one unit, essentially like a giant bullet. Yet unlike simple bullets, the propellent cartridges are made to detatch from their shells when necessary. However, the two components should never be loaded separately and must be rejoined before use. Otherwise, firing the gun will likely destroy it and potentially lead to injury or death among the crew.

An essential characteristic of a howitzer is the ability to vary the charge (i.e. the amount of explosives) behind each shell in order to fine-tune their angle of fire. Thus, the propellent inside the cartridge of an M117 round is contained in a number of color-coded cloth bags. The 'blue charge' (Charge 3) and the 'white charge' (Charge 2) are arranged beside one another, sitting atop the larger 'red charge' (Charge 1) which fills the bottom portion of the casing. However, there is a total of five propellent bags: the blue & white charges each consist of two smaller bags stacked on top of one another. Commands for the propellent load behind a shell are given as follows, according to which charge(s) should be removed:


 * "Full Charge!" - no propellent is removed and the round is loaded as is, with all five bags.


 * "Charge 3!" - round is loaded with both blue bags having been removed


 * "Charge 2!" - round is loaded with both white bags having been removed


 * "Charge 1!" - round is loaded with only the large red charge remaining

The division of the blue and white charges each into two separate bags allows for further specificity:


 * "Charge 3.5!" - [spoken as "Charge three-five!"] round is loaded with only one blue bag having been removed


 * "Charge 2.5!" - [spoken as "Charge two-five!"] round is loaded with only one white bag having been removed


 * "Charge 1.5!" - [spoken as "Charge one-five!"] round is loaded with one blue & one white bag having been removed

Furthermore, an M117 gun-howitzer is normally issued with a small number of APDS - or "Armor-Piercing, Discarded Sabot" - rounds for rare and dire circumstances when its position is being threatened by enemy armored fighting vehicles, which its crew will have to engage with direct fire over open sights (i.e. for emergency situations where the artillery piece must be used as a direct-fire anti-tank gun in self-defense).

Description of the U.S. APDS Round
An American large-calibre APDS round - as would be utilized by the M117 gun-howitzer - consists of a truncated shell without a nose cone (resembling a tapered cylinder or a flower pot), which is made of a special light-weight / high-strength alloy and thus has significantly less mass than would be normal. This specialized shell (called a 'sabot') contains a smaller-diameter projectile, resembling a proper bullet, with a long and sharply-pointed nose. Said sub-projectile is only a little bit more than half the diameter of the enclosing shell, but also roughly 1/3rd again its length - and so protrudes from it by a significant degree. The sabot / outer shell has seams (it is not a single piece), being held together by pins at its base and a band of tooth-like links near the top. When the round is fired, the friction of its acceleration shears said links apart: the shell then simply breaks up as soon as it leaves the muzzle. In other words, once the gun-barrel is no longer compressing the sabot together, the pins are flung from their sockets by its rapid rotation and it separates into sections - which then fall to the ground due to aerodynamic drag while the projectile inside continues on toward the target.

The actual APDS projectile - called the "penetrator" - is made of extremely dense and strong tungsten-carbide with an outer sheath of thin steel resembling a bullet. It pierces armor through raw velocity and force of impact, concentrated into a small point for greatest possible effect. Encasing it in a lightweight sabot provides sealing contact with the barrel to impart stabilizing spin while also maximizing the surface area upon which the exploding propellent can act, resulting in an accurate trajectory and maximum velocity.

Usage with the M117
An M117 crew will normally prepare their APDS rounds in advance - usually while setting up their artillery piece - in order to have them on-hand and ready for use at a moment's notice, as the process is rather time-consuming. Each of the special shells comes pre-packaged in a metal canister along with two additional propellant bags: "Super Charge" and "Super Charge Plus", colored purple and black respectively. To make an APDS round ready for firing, the artillerymen must take a normal round of ammunition, detatch the cartridge, set the shell aside, remove both 'blue' and 'white' propellent bags (i.e. Charges 2 & 3) from said cartridge, replace them with the two "Super" bags and then affix the APDS shell to the modified propellent casing. Normally, battalion ammunition personnel will issue each battery an extra set of cartidges for the discarded conventional shells so that they do not go to waste.

Crew and Tactical / Battlefield Organization
The crew of each M117 - and all American artillery pieces in general, with varying numbers of men - is divided into three positions: the Section Chief (a Sergeant) who commanded the artillery team, gunners and cannoneers. The man most responsible for actually aiming the piece is the Gunner-Corporal, who is aided by an Assistant Gunner (called the "Number 1"). The M117, like most U.S. artillery pieces, uses a two-man laying system which allows aiming and elevation setting to be done simultaneously (most other countries use a one-man laying system where-in all the work is done by the artilleryman equivalent to an American Gunner-Corporal).

Thus, as the firing command is being given by the Sergeant, the Gunner-Corporal - positioned on the left - operates the primary telescopic sight (known as the "Gunner's Quadrant") to bring the muzzle into alignment as ordered. The Number 1 / primary Assistant Gunner on the right simultaneously turns the elevation wheel to set the artillery piece at the proper angle and works the lever that opens the breech-block.

The remainder of the crew - an additional two Assistant Gunners and six Cannoneers - are responsible for ensuring the properly-charged shell is used and for getting it in the breech. Two Cannoneers run forward immediately to the artillery piece as the firing command begins and act as loaders, while the other four facilitate the transfer of ammunition from its position near the gun into the hands of said loaders. The two junior Assistant Gunners are responsible for "managing" the ammunition: making sure that the proper shell is passed up to be loaded, setting the fuzes, etc. Once the shell is in the breech, the loader on the left side throws the lever that engages the pneumatic rammer and pushes the round snugly into place. He then reverses said motion, retracting the ram to its original position and also automatically closing the breech.

With the Gunner-Corporal's report of "Ready!" - signaling he has sighted the gun - the Sergeant holds up his right hand. The loaders move away from the recoil path of the breech. His hand drops, he shouts "FIRE!" and the senior Assistant Gunner pulls the firing lanyard. Upon firing, the breech automatically re-opens and spits out the spent propellent cartridge. The loading process is repeated to desired effect.

Operational Employment
The M117 is the principle artillery piece of the American military, issued at the Divisional level. American forces do not employ full-calibre "field guns" and as such this is the sole non-self-propelled weapon operated by U.S. Artillery Battalions. It is used almost exclusively for indirect fire missions from well behind the front lines, and is always accompanied by an M78 Armored Half-track. Although primarily a Howitzer designed to fire low-velocity ammunition at a high ballistic arc, it may also be used as gun and fire high-velocity shells across a shallow trajectory, being is equally effective in both roles.

Compared with its M114 predecessor, the M117 is lighter, has greater range, better accuracy and a higher rate of fire. With its new gunshield, it is also better protected, while the higher-velocity cartridge rounds and new gunshield allow it to be used as a reasonably effective direct-fire weapon in a pinch: making it considerably more survivable. Its principle disadvantage when compared to the older weapon system is the weight of its longer, fixed metal ammunition, which is more difficult to load (but faster, as separate bagged propellent does not have to be inserted). Despite the single-piece nature of the new shells, two loaders are retained due to their length and weight in order to compensate for increased crew fatigue.

In many ways, the M117 is more similar to its Russian analogue, the D-20 152mm gun-howitzer. Both weapons are technically "gun-howitzers", atlhough the American artillery piece gives preference to the "howitzer" mode of operation. Compared with the Russian piece, the M117 has a higher rate of fire and is also more accurate, but its range is somewhat less.

Production History
Designed: 1957

In Service: 1958-Present (Primary Artillery Piece since 1960)

Manufactuer: United States Ordnance Department, Rock Island Arsenal

Specifications
Weight: 4,500 kg / 9,921 pounds

Length: 8.71 meters / 28.58 feet

Barrel Length: 36 calibres (i.e. 36 multiples of 155mm); 5.58 meters / 18.3 feet

Shell: 155mm HE, HEAP, Fragmentation, Illumination, smoke, chemical, APDS

Calibre: 155mm / 6.1"

Breech: Semi-automatic horizontal sliding block with integral pneumatic ram

Carriage: Split trail

Elevation: -2º to +63º

Traverse: 25º

Rate of fire: 8-9 [or more] rpm burst, 4-6 rpm prolonged, 60-70 rph [rounds-per-hour] sustained


 * note: exact rate of fire varies considerably depending on the experience of the operators, the conditions on the battlefield, the particular fire mission being executed and the level of wear / fatigue on the artillery piece itself

Effective Range: 15,200 meters

Maximum Range: 20,000 meters

Advantages

 * The M117 weighs considerably less (4,500 vs 5,700 kg / 9,921 vs 12,566 lbs)


 * The M117's better rifling and electroplated chromium barrel liner gives it a significant advantage in accuracy. The chrome-plating, specifically, makes the barrel better able to withstand firing wear - which, over time, degrades the rifling and thus the accuracy of the piece. The M117 also has a slightly longer barrel, perportionally, than the D-20 (36 calibres vs 34), although the effect of this on its accuracy is negligable - if even there is one at all.


 * The horizontal sliding block breech - versus the D-20's vertical sliding block - requires less energy to open or close and the required force remains constant at all angles of fire. Since both the U.S. and Russian pieces use automated, mechanical actions to open their breeches, this results in a small but detectable and potentially significant increase in the speed and smoothness of the M117's breech operation.


 * The M117's single-piece cased ammunition is much more quickly (if not so easily) loaded, as it is all inserted into the breech at once. The D-20, by comparison, also uses metallic cartridges but they are separate; the propellent case is loaded after the round in an independent action. This means that the reloading cycle of the M117 is shorter.


 * The M117 has an automatic pneumatic rammer built into the loading tray, which means the crew does not have to manually, physically ram the ammunition into place after loading. This further decreases the howitzer's reload time, improving its rate of fire. In the event that the mechanism fails, the crew also carries an old-fashioned manual rammer.


 * As mentioned above, each M117 is supplied with a small number of APDS (Armor-Piercing, Discarded Sabot) rounds and special "super-charge" bags for the propellent cartridges to increase projectile velocity. These come pre-packaged, with one box holding an APDS round as well as a "super-charge" and a "super-charge plus" propellent bag. Each gun's alotment of these special rounds is normally pre-configured as the artillery piece is being set-up and kept on-hand for quick access. Said APDS ammunition, while complex and time-consuming to configure, allows M117s in danger of being overrun by enemy armor to effectively engage and destroy the attacking tanks (as long as they don't run out of APDS rounds).

Disadvantages

 * The M117's range is less than that of the D-20 (15,200 meters effective and 20,000 meters maximum vs the D-20's 17,400 meters effective and 24,000 meters maximum).


 * The D-20s rounds, due to its vertical sliding block breech, are more easily loaded because the top of the breech-block acts as a loading tray. This advantage is not really significant, however, in the face of the M117's numerous other rate-of-fire improving features.


 * The M117's ammunition is quite unwieldly and heavy, requiring two loaders. Normally, each gun maintains two pairs of loaders who "tag-team" in intervals to ward off exhaustion during sustained firing. The Russian D-20, meanwhile, uses two-piece ammunition with separately-loaded cartridges, which are of different lengths for different levels of propellent charge: meaning varying the amount of explosive behind each shell is a simple matter of selecting the proper cartridge. In the case of the M117, the charge casing must be physically detatched from the shell, color-coded propellent bags inside removed to the desired charge level, and then the casing re-attached to the round for loading. Thus, M117 crews normally keep a supply of pre-configured rounds on hand with one or more charge bags already removed. If at all possible, the U.S. artillerymen endeavour to deploy at such a range or in such a position that they may utilize the maximum charge, so that no propellent has to be removed - saving time. This is not, however, always possible, and in these cases the "runners" who ferry ammunition from the dumps to the actual artillery pieces must take the time to modify the charges of the shells before handing them off to the gun crews. U.S. organizational systems for artillery batteries and battalions include extra personnel in order to ensure this process is facilitated smoothly without loss of time, but this obviously creates additional levels of logistical complexity (not to mention uses up more manpower).


 * The Russian D-20, due to the above factors, is manned by a crew of 8, while the M117 is fielded with a crew of 11. (Note that these standards do not represent the minimum number of trained personnel required for effective operations of either weapon).