Munitions -155mm Howitzer, Medium, Towed

The Projectiles

The projectiles of the 155mm Howitzer went by many names; shells, projos, rounds, and joes to name a few. Just as there were many names, so there were several different types of projectiles to accomplish the many missions the gun was expected to perform. Add several different fuzes, and two different powders, and the 155 became a very versatile weapon.

The first, and most utilized projectile, was the M107 High Explosive round (HE). With the superquick, point detonating fuze(PD), the round was used against personnel, and light armor. With the turn of a screw,the fuze became delayed, and an HE round could dig out bunkers, and other fortified positions. With a Mechanical Time, or Variable Time fuze, the HE would airburst with devastating effect to troops in the open or even in trenches and foxholes.

To mark where the explosive rounds would hit, or to indicate to a unit in the field where they were, the M116(A1) Colored Smoke/High Cloud round was used. The smoke was bright red, purple, or yellow. The high cloud round was white smoke, but could be used as a marker at night as it left a stream of sparks going into the target. All of the smoke rounds used mechanical time fuzes.

The White Phosphorus (WP), M110, was also used as a marker round. It could be fitted with PD, VT, and MT fuzes. When the situation called for it, white phosphorus became a devastating weapon against personnel. The thick white smoke could be used as a screen to mask movement by troops in the field.

Illumination was a very important mission for artillery in Vietnam. To accomplish that, the 155mm howitzer used the M485(A1). Each round was capable of lighting up a 1000 yard diameter area. The flare was ejected at 700 yards altitude, and would stay lit for at least 90 seconds. By keeping two flares in the air at a time, shadows were reduced. At a firing rate of two rounds a minute, an area could be kept lit all night long.

The ICM (Improved Conventional Weapons), or Firecracker round (M449) was used for antipersonnel missions. An airburst, base ejecting shell, it dropped sixty bomblets that bounced up five feet in the air before going off. It proved to be extremely effective against enemy in the open, or in positions with little, or no overhead protection.

We have included the gas round, M121A1 although it is not clear that the 1/92nd ever fired it in anger. Delivered as an airburst, or as a point detonating round, it contained Ca, or Cs types of "tear" gas.

Shell	M #	Color	Stripe/ Color	Lettering Color	Weight in Pounds
High Explosive	M107	Olive Drab	None	Yellow	97
Gas	M121A1	Gray	Three Green, One Yellow	Green	100
WP	M110	Lt. Green	One Yellow	Red	98
Smoke/HC	M116(A1)	Lt. Green	None	Black	86
Illumination	M485(A1)*	Olive Drab	One White	White	85
ICM	M449	Olive Drab	One Row Yellow Diamonds	Yellow	95

* The M485 was to be fired at a maximum charge 6, while the M485A1 could be fired at Charge 7.

The Fuzes The fuzes were as varied as the projectiles. Depending on which round was used, and the mission it was to perform, several of the rounds took two or more fuzes. Here are the fuzes, and what they were used for.

The M557, or M739 superquick, point detonating fuze had a dual purpose. Put on an HE, or WP round, it would detonate upon impact. With an aluminum filled body, the M739 needed no rain caps to prevent premature fire in inclement weather. A quarter turn of the side screw would delay the fuze so that a round could penetrate to destroy bunkers, and other dug-in positions. The PD fuze came with a booster charge to accelerate detonation of the projectile.		The M564 is a mechanical time/superquick fuze for use with detonating rounds such as the HE, or WP. It can be set for times from 2 to 100 seconds, and will explode upon impact, or set time depending on which comes first. Fuzes manufactured through 1969 had to be set on 90 seconds time for the SQ function to work. Premature detonation could occur during rain


The M278 Variable Time fuze (VT) is a proximity (20 meters) fuze also used with detonating rounds like HE, and WP. It could be set from 5 to 100 seconds. The M278 was, in effect, a self contained radar unit. The setting on the time ring determines where along the trajectory the fuze will become activated. It also has an impact element, and will detonate either on impact, or proximity, which ever comes first. The nose is painted black to reduce static electricity.		The M565 fuze is much like the M564 except it lacks the point detonating assembly, and booster cup. Like the M564 it can be set from 2 to 100 seconds, and uses a Vernier scale to achieve an accuracy of .1 of a second. This fuze is used on base ejection rounds only.


The M501(A1) is a mechanical time, superquick fuze. It can be set from 2 to 75 seconds. It is noted in a 1965 training manual that the M501(A1), when set for time, will fail 20% of the time it is fired with charge one green bag, and 10% of the time at charge two green bag. It will function upon impact. The only round the 155mm Howitzer fired with the M501(A1) was the High Cloud/ Smoke.		The M78(A1) fuze with the M25 booster were constructed especially for use against concrete targets. A non delay type was painted white about one inch down from the nose of the fuze. It was used for marking concrete targets. The Booster M25 is a separate assembly designed for use with the M78(A1), and cannot be used with any other fuze. It is shipped with the M78(A1) for assembly to the projectile before it is fired. If used in a deep well round, the supplementary charge must remain in the well.

Fuze	Type	Uses	Projectiles
M557/M739	SQPD/Delay	Impact/Subsurface	HE,WP,Gas
M564	MT/SQ	Airburst/Impact	HE,WP,Gas
M565	MT	Airburst	Illumination, M449
M501(A1)	MT/SQ	Airburst/Impact	High Cloud/Smoke
M729	Proximity/SQ	Airburst/Impact	HE, WP, Gas
M78(A1)	CP/Delay	Concrete	HE, WP, Gas

Fuze Wrenches, and Setters The fuzes were as varied as the projectiles. Depending on which round was used, and the mission it was to perform, several of rounds took two or more fuzes. Here are the fuzes, and what they were used for.

In order to tighten the fuzes to the projectiles, and set times on the various time fuzes, a number of wrenches, and setters were used. The simplest device was the M16 "J" wrench. It could be used to tighten rounds to projectiles, and even used to set times on the fuzes, although this practice was frowned upon by command. A fact known all too well by this webmaster.		The M18 fuze wrench was the workhorse. Used to tighten fuzes to the rounds, the handle also had a straight slot screw driver that was used to set the PDSQ fuze to delay.


The M34 fuze setter was one of the simpest to operate. Just slide the wrench over the fuze, and turn clockwise to set the time. Should you make a mistake you can go around again, and using the vernier scale on the fuze you can set to an accuracy of .1 of a second.		The M26, and M28 fuze setters were very similar hand operated, dialed insturments. The M26 had two scales, 0-25, and 0-75 seconds, while the M28 had 0-45, and 0-100 second scales. Both had a handle assembly for setting a specific time value; a pointer, dial time scales, and body assembly contured to fit the shaped of the nose of the fuze, and containing the setting pawl which engages the setting groove of the fuze. There is a base plate assembly which serves to stop the setter at the desired time automatically. The M26 used luminous numbers on its dial for night use, while the M28 had a button controlled lamp in its handle assembly. Both had a thumbscrew to hold the dial at the proper setting.