History of the 4.2" Chemical Mortar

In 1959, as one of its publications on the technical services in WWII, the Army's Office of Military History published a 3-volume history of the Chemical Warfare Service (CWS). Volume I was Organizing for War; volume II, From Laboratory to Field; and volume III, Chemicals in Combat. Chapter VI of Volume II is entitled "Chemical Mortars and Shells" and is divided into five sections, all of which are extracted below in their entirety.

The 4.2-Inch Chemical Mortar
Increasing the Range in World War II
Mortars of Unusual Design
Mortar Shells
Mortar Gunboats

The 4.2-Inch Chemical Mortar

Weapons available to American ground troops for delivering toxic agents included Livens projectors, grenades, land mines, mortars, rockets and artillery shells. If gas warfare had broken out, the burden would have fallen chiefly on the 4.2" chemical mortars of CWS mortar battalions.

The 4.2" mortar descended from the old Stokes mortar of the British Army. Britain invented the Stokes in World War I to overcome the disadvantages of gas cloud attacks. Gas clouds could be tremendously effective under the proper conditions, but they required considerable labor, were wholly dependent on the weather, could only be used with a few gases and, by their color and odor, sometimes warned the enemy. The Stokes had a smooth-bore barrel and therefore could not fire shells with pin-point accuracy. On the other hand, it had certain advantages. Troops could easily move it and fire shells at the rapid rate of twenty a minute. Each shell held more than two quarts of toxic agent. Because of these factors a mortar could suddenly overwhelm an enemy position with a large amount of poison gas.

The First Gas Regiment of the CWS obtained Stokes mortars from the British in 1918 and employed them along the western front. In July 1918, the Army contracted with American firms for the manufacture of these mortars. More than 400 were turned out but they did not reach France in time for battle.

The CWS found the Stokes mortar a versatile, useful weapon, and in the early 1920s set out to lengthen its range. The objective, as laid down by Maj Gen Fries [then the Chief Chemical Officer of the U.S. Army], was to double the WWI range of 1100 yards. Early experimentation showed that heavy powder charges could hurl the shell only a few hundred yards beyond the normal range. This was dangerous since the higher pressure within the mortar at the instant of explosion could burst the barrel. The designers attached fins to the shell, enabling it to fly through the air like a dart, and shot it 2,600 yards. But the shock from the exploding propellant generally damaged the fins, and the shell's flight was short and erratic. In 1924, Captain McBride and Dr G.S. Maxwell rifled several barrels with varying pitches and numbers of grooves. During machining operations, metal was gouged out of the bore, increasing its diameter from four to four and two-tenths inches between lands. This marked the end of the old smooth-bore Stokes mortar and the beginning of the new 4.2" chemical mortar. On 7 June 1924, one of the experimental barrels sent three shells through the air on accurate, spin-stabilized flights of almost 2300 yards.

Adoption of a rifled barrel made it necessary for engineers to redesign each component of the mortar, from baseplate to shell fuze. WWI shells had an all-ways fuze to make certain that the tumbling shell would explode no matter whether it landed on its base, side or nose. Fuzes of this type could not be used on a spinning shell since centrifugal force would activate the fuze and cause the shell to burst as it left the muzzle of the mortar. After considerable experimentation, engineers developed a safe, dependable fuze that could be set for impact or time.

Something had to be done to prevent liquid fillings, such as mustard or phosgene, from surging around inside the shell, unbalancing it and causing it to tumble and yaw in flight. This characteristic had not mattered with the Stokes mortar since its shells had tumbled anyway, but it affected the accuracy and range of the rifled mortar. Technicians solved this problem by fastening vanes inside the shell to swirl the fillings as the shell spin through the air.

To seal the bore against loss of explosion gases, and to force the shell to rotate as it sped up the barrel, the men had to devise a driving mechanism for the base of the projectile. This consisted of two round plates, one of brass and one of steel, the brass disk designed so that its edge could be forced outward by pressure. When the powder exploded, gas pressure rammed the steel plate up against the softer brass plate, forcing its edge out and into the grooves, sealing gases in and forcing the shell to spiral out of the barrel.

For the baseplate of the Stokes mortar it had been feasible to have a steel cup, bolted to an oak plank. But recoil from the new 4.2" barrel soon pounded this type of baseplate into splinters, and a forged steel baseplate had to be produced. Finally, in 1928, after several years of experimentation, model M1 of the 4.2" chemical mortar was ready for service.

During the next decade CWS engineers put considerable thought into improving the standard model. The practice of digging an emplacement, which took time and reduced the mobility of the mortar, was abandoned and the base was placed directly on the ground. The two-legged support inherited from the Stokes mortar was improved and retained for a time, but it proved to be so awkward that it gave way to a single leg. Engineers then found it necessary to place connecting rods between the baseplate and barrel support to keep the recoil from forcing base and support apart. The barrels, hitherto obtained from the old Stokes mortars, were made specifically for the new model from seamless drawn-nickel steel tubing. A spring shock absorber was placed on the barrel to prevent the force of recoil from breaking the connection between support and the barrel. At last, after seven years of work, the CWS completed a greatly improved mortar, model M1A1, with a range of 2400 yards. This mortar was in the hands of chemical troops at the time of Pearl Harbor.

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Increasing the Range in World War II

The next step in the development of the mortar came about as a result of an addition to the mission of CWS troops. Up to the time of America's entrance into the war, the mortar had been considered as a weapon for firing toxic agents, smoke and incendiaries at the enemy. In April 1942, General Porter [then the Chief Chemical Officer] asked the Services of Supply for permission to use high explosive ammunition in the mortar. Chemical troops had fired HE in WWI and, if allowed to do the same in WWII, it would broaden their usefulness in the theaters of operation. The SOS gave its consent, thereby giving the CWS impetus in lengthening the range.

The mortar had been developed under the pre-war doctrine that chemical shells would be employed only within a range of 2400 yards. This concept did not apply to HE, and the CWS set about increasing this distance before the mortars saw action. Engineers could have lengthened the range by redesigning parts of the mortar, but such a step would have taken time. When tests demonstrated that 50% more powder hurled the projectile an additional 800 yards, bringing the total distance up to 3200 yards, a larger charge was adopted. The higher explosion pressure imposed more strain on the barrel and baseplate than they had been designed for. To prevent accidents, the service adopted a tougher barrel and baseplate. To prevent mortar squads from using the old barrel and perhaps blowing themselves up the CWS designated all mortars with the new barrel as model M2. The CWS carried the M2 into all theaters and some were still in action at the end of the war.

The 4.2" mortar first saw action [with the 2nd and 3rd Chemical Mortar Battalions] in the taking of Sicily in the summer of 1943. Mortar squads were among the first waves of troops to hit the beach, and they went into action a few minutes after landing. During the 38-day campaign, they shot 35,000 rounds of ammunition in crash concentration, harassing, interdictory and counter-battery fire, and in tactical smoke screening missions. The mortar made an excellent impression on commanders of infantry, ranger, armored and airborne units. Thereafter there was no question that the CWS had taken the right course in turning the chemical mortar into a HE-delivering weapon.

After troops tried out mortars in Europe, they began calling for a longer range. Back in the U.S., the CWS had already anticipated the demand and had succeeded in adding another thousand yards to the flight of mortar shells. It had achieved the increase by changing the form of the propellant so that it burned slowly, gave off gas more evenly, and thereby became more efficient. Lt. Arthur Denues had experimented with the propellant, trying different shapes, arrangements and types, and had finally found out that with disks of powder of a certain thickness, the range depended upon the number of disks.

The minimum charge, which lobbed the shell only 340 yards, could be lengthened to 4400 yards simply by adding more disks. The maximum gas pressure did not become excessive and there was no disturbance in the ballistics of liquid-filled shells. The disks were cut square, with a hole in the center to allow the disk to slip on the cartridge container. Sufficient disks, sewn together in bundles of different thickness, were placed on each shell before shipment to give a range of 4397 yards. Before the shell was fired, the mortar squad could remove one or more disks to shorten the range.

Notwithstanding that the range of the mortar had almost doubled by 1944, troops in the field were still not satisfied. They wanted the weapon to hit targets 5000 or more yards away. One means of accomplishing this was to devise a jet accelerator that would fit on the base of a shell and give it a boost after it left the barrel. The CWS started work on such a device but soon canceled the project after a survey showed that development would take too long, and men with the know-how could not be spared from other mortar projects.

Two other courses lay open to the CWS, a long-term project to completely redesign the mortar from baseplate to standard, and a short-term project, which might not work, of again modifying the propellant. Experiments with the propellant began after calculations and preliminary experiments indicated that the velocity of the shell could be increased 100 feet per second and the range jumped to more than 5000 yards if the chamber volume of the mortar and the weight of powder were doubled.

Engineers set up an experimental mortar and made test firings, but to their surprise the shells had an unexpectedly short range. They carefully checked all possible sources of error and at last discovered that the high pressure from the explosion had deformed the rotating disk on the base of the shell and this had increased air resistance. Development of a new rotating mechanism proved to be the major task in extending the range, but when the job was finished the mortar shell carried distances of 5600 yards in firings at Edgewood in June 1945. These results obtained in the shop and on the test field occurred too late in the war to be transmitted to the battlefield.

Regardless of demands for longer ranges, a complete redesign of the mortar was necessary by 1944. By this time, both the M1A1 and M2 had been used extensively, particularly in mountainous regions where artillery found it difficult or impossible to operate. The terrain in which mortars operated in Italy and the Southwest Pacific was at times so rugged that standard mortar carts or jeeps could not be used, and the CWS had to devise a mule pack.

This constant use of mortars took its toll in worn and damaged parts, much harm being caused by attempts to get more service out of the mortar than it had been designed to deliver. The CWS Technical Division set out to produce an entirely new mortar that would be free from the limitations inherent in the basic design of the M2 model which had been intended originally to fire a range of 2400 yards. By the end of the war, an experimental model, E37, had been constructed and test fired at Edgewood. After the war, the service continued development until 1949 when the War Department, feeling that the mortar was now a legitimate weapon of the infantry, transferred the responsibility for development to the Ordnance Department. The Ordnance Department made a few final modifications and standardized the mortar as model M30 in 1951.

American troops who saw the chemical mortar in action in the Pacific and in Europe had a high opinion of the weapon. German and Japanese troops respected its fire power and accuracy. Generalleutenant Ochsner of the chemical warfare branch of the German Army stated that "from the technical point of view the American 4.2" chemical projector is very good; the construction is simple, it is a very handy weapon in battle and its firing efficiency is high."

The German Army had two types of chemical mortars, the smaller with a caliber of 8cm (approximately 3.2") and the larger of 10cm (3.9"). The 8cm mortar was smooth-bore, like the old Stokes, and was similar to the 81mm issued by the Ordnance Department to American infantry. The complete weapon weighed 125 lbs and had a range of 450 to 1300 yards. German mortar shells were pear-shaped with fins fastened to the tail to stabilize their flight. The 10cm mortar was more complicated than either the 8cm or the American 4.2" in that it was a breech-loading weapon with a hydro pneumatic recoil mechanism. It was mounted on a two-wheel chassis and weighed approximately a ton, six times heavier than the 4.2" mortar. The heavy weight and the wheeled mount greatly restricted the mobility of the weapon, a distinct disadvantage in comparison to the easily transported 4.2" mortar. The maximum range was 6800 yards.

The principal Japanese ground weapon for the employment of chemical munitions was the 90mm (3.5") mortar. This was a smooth-bore weapon that fired a dart-shaped, fin-tailed shell. Its total weight was 350 lbs, slightly heavier than the 4.2" mortar, and its maximum range was about 4200 yards.

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Mortars of Unusual Design

While the 4.2" mortar was the workhorse of CWS troops during WWII, members of the technical staff experimented with other models. Following a suggestion of the Chemical Warfare Board in 1943 that fewer mortars would be needed and targets could be changed more rapidly if the weapon could swing through a full circle, engineers began development of a special mortar with a traverse of 360º.

They designed several models, one or more of which might ultimately have been satisfactory, but finally dropped the project when the AGF [Army Ground Forces] decided that a mortar with such an extreme traverse was unnecessary. Along with the 360º mortar , the men experimented with a mortar of 90º traverse but the weapon was not particularly successful and the project ended with the war.

To simplify the training of mortar squads, the Chemical Warfare Board devised an ingenious subcaliber mortar consisting of a steel pipe that fitted inside the standard mortar barrel. The shell was fashioned of steel and wood and could be used over and over. The weapon was fired in the usual manner and ranged from 115 to 400 yards. S mall glass bottle of smoke solution could be fitted to the nose to give the appearance of a miniature explosion when the shell landed. Not the least of the merits of this subcaliber mortar was that it dropped the cost of firing a training round of ammunition from several dollars to thirteen cents.

In 1943, at the suggestion of Col. William A. Borden, Ordnance Department, who had conducted a survey of munitions in the Pacific Island, the CWS began to develop a mortar which could fire horizontally into low Japanese bunkers of coconut logs and earth. The standard mortar barrel could not be dropped below an angle of 45º with the horizon, and a way had to be found of holding the barrel level with the ground and at the same time neutralizing the tremendous recoil of a quarter of a million pounds. Engineers at the CWS Laboratories at Columbia University devised a method of leveling the mortar by a new baseplate that could be bolted or chained to the trunk of a tree.

While this solved the recoil problem, as long as mortar companies fought in wooded areas, it did not end the development work. When a mortar fired on a flat trajectory, for which it had never been designed, unusual strains were set up in the weapon. These had to be overcome by redesigning certain components. Even the shell was modified and its weight increased. Tests proved that the new model would be satisfactory in the field, but the weapon was not carried beyond the experimental stage because a recoilless mortar, which the CWS had started to develop at almost the same time, offered a better possibility of solving the multitude of problems that accompanied the change from angular to horizontal firing.

A recoilless gun was open at both ends instead of one end as in the conventional gun. When the propellant charge exploded, the shell sped forward through the barrel while the gas blew backward through the breech. The gun, shell and propellant were designed to make the forward action equal to the backward reaction, eliminating recoil.

Recoilless weapons had been investigated by the American Army in WWI. In WWII the idea was revived by the Germans, the British and then by the Ordnance Department. When the CWS began development of a horizontal mortar, it realized that the principle involved in a recoilless gun might be applied and, in October 1943, Gen. Kabrich asked Dr. C. N. Hickman, chief of section H, division 3, NDRC, to undertake the development of a mortar having no recoil. That same month, the first recoilless attachment to fit on the breech of the standard mortar was designed and, in November, firing trials were started.

Step by step, the ignition system, firing mechanism, reaction chamber and shell were perfected. The shell was fired in the usual manner. A small rocket, called a rocket driver by the designers, was attached to the fuze. When the mortar was fired, the rocket driver hurled the shell back into the barrel where it struck the firing pin. The firing pin then detonated the propellant charge and started the shell forward. The rocket driver fell off while the shell was in the air, exposing the fuze.

By August 1944, the model was ready for a full-scale demonstration at Edgewood. Service officers were so impressed that they ordered the gun completed under top priority. The final, standard model consisted of a two-piece barrel mounted on a caliber .30 machine gun tripod. Targets 3800 yards away could be hit but accuracy was best below 1000 yards. The 28-lb HE shell easily demolished replica Japanese bunkers of earth and logs.

The CWS contracted for the manufacture of 1000 recoilless mortars, 100 of the weapons being completed before the end of the war. Recoilless mortars reached the Tenth Army in the Pacific theater in time for the Okinawa campaign, but otherwise they were produced too late for battle use.

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Mortar Shells

The 4.2" mortar shell with fuze weighed 18 lbs and held approximately 7 lbs of chemical agent. At the beginning of the war, the authorized toxic fillings were mustard, lewisite, phosgene, CNB (a solution of chloroacetophenone in benzene and carbon tetrachloride) and CNS (a solution of chloroacetophenone and chloropicrin in chloroform). Later, cyanogen chloride was added. Some measure of the service's opinion of the relative value of different toxic agents for gas shoots may be gauged from the number of shells filled with various agents from 1940 to 1946: 540,746 contained mustard, 49,402 phosgene, 41,353 CNS, 12,957 CNB, and 175 lewisite.

The German army came close to the American in its stockpile of mustard-filled shells, some 400,000 10-cm mustard-arsenöl shells coming to light after the war. No other chemical mortar shells were found. They either may have decided that other fillings were not satisfactory or else they were not able to put other shells into production.

The Japanese had a variety of toxic-filled munitions for use with their 90mm mortar. One type of shell held a half-pound of diphenylcyanoarsine, a vomiting agent, and a pound of TNT. The TNT scattered the agent in aerosol form besides acting as a high explosive. Another shell held two pounds of blistering agent, generally a 50-50 mixture of lewisite and mustard. Shells containing hydrogen cyanide were also reported among Japanese supplies.

American mortar shells held more agent than either Japanese or German shells. They contained from 6.25 to 7.56 lbs of chemical, depending upon the physical characteristics of the shells. Thus the total weight of filled shells varied from 24 to 26 lbs. In terms of percentage, the agent comprised from 26 to 30% of the shell. In contrast, German 10-cm mustard-arsenöl shells weighing 15 lbs held 3 lbs or 20% of agent, and Japanese 90-mm shells weighing 12 lbs held 2 lbs (17%) of vesicant agent, or ½ lb (4%) of vomiting agent. Shell for shell, the 4.2" mortar could have laid down a heavier concentration of gas on an area than the enemy mortars.

The possibility that chemicals would be used during the war was the reason for the mortar's presence among CWS troops; yet chemical shells, with the exception of smoke shells, were never fired. It was the HE shell which gained for the mortar the high regard of infantrymen. When the CWS received permission to use HE, munitions experts modified the standard shell slightly, removed the vanes and burster tube, and then loaded the missile with TNT. An HE shell with fuze weighed 22 lbs and the explosive charge weighed 8 lbs. This quantity of TNT, approximately one-third of the weight of the filled shell, represented a high loading efficiency, and the blast and fragmentation effect of the shell upon impact was tremendous. In September 1942, the CWS standardized this shell as model M3.

During the course of development work, the CWS Technical Command produced several experimental mortar shells, one of which, a high explosive fragmentation shell with good flight characteristics and ballistic properties, was superior to the M3 shell in demolishing Japanese-type pill-boxes. The shell wall was twice as thick as that of the standard HE shell, while the filling of TNT was a bit less, 6.6 lbs. In November 1943, a CWS representative returning from the Southwest Pacific area reported that such a shell was needed immediately. The CWS designated the new shell as model M4 and ordered a large supply.

In the spring of 1944, the Field Artillery Board compared the new M4 with the older M3 in a series of tests against targets in shelter trenches and open fields. In percussion, ricochet and low-angle time fire, shell fragments from the M3 scored more hits than fragments from the M4. In high-angle time fire, both projectiles produced about the same number of hits. As a result of these tests, the CWS halted the production of the new shells, and thereafter used them only when there was a shortage of the M3 shell. About 67,000 new M4 shells were manufactured, in comparison with the more than 6,400,000 of the M3 type.

Smoke shells made up a large fraction of the service's output of mortar ammunition. Authorized smoke fillings included white phosphorus (WP), a solution of sulphur trioxide in chlorosulfonic acid (FS) and titanium tetrachloride. "The American white phosphorus ammunition was outstandingly good," wrote Generalleutenant Ochsner after the war. These shells threw up a large volume of dense white smoke that was useful as a marker or as a smoke screen. Burning chunks of phosphorus flying through the air frightened enemy soldiers. Phosphorus could ignite dry underbrush, hay, paper and other combustibles and thereby serve as an incendiary. And, finally, the agent could cause casualties among enemy troops by inflicting burns. Mortar squads fired quantities of WP second in volume only to HE. Over three million WP shells came from filling plants in the United States, more than all other mortar shells, excluding HE, combined. In comparison, the service procured only one-third of a million FS smoke shells, and none containing titanium tetrachloride.

The German army would have been happy to have had the same plentiful supply of WP as the American army, but Germany lacked the raw materials for producing phosphorus, and its army had to depend on inferior Berger mixture or on sulphur trioxide.

In addition to chemical, HE and smoke shells, the CWS developed other types of mortar ammunition for special purposes. In 1943, the Chemical Warfare Board suggested that an incendiary shell might be useful in driving enemy soldiers from wooded positions or combustible buildings. After a preliminary study, the question arose as to whether such a shell would be worth the time spent on it. The CWS canvassed chemical officers in the various theaters and found only one who thought that the shell might be useful. As a result of this survey, the project was dropped. But, in the spring of 1944, the service revived the idea when the European theater showed interest in the incendiary shell. Munitions engineers at CWS designed a base-ejection shell holding four hollow magnesium incendiary cylinders filled with thermite mixture. The shell acted like a small mortar. As it struck the target, an explosive charge in the nose blew the burning cylinders backward through the tail, and the magnesium in turn set fires in the target area. This shell was just being perfected as the war ended.

In March 1944, the CG, Central Pacific Area, requested an illuminating shell. The CWS had had no experience with this type of projectile but both the Ordnance Department and the Navy had shells which, upon bursting, released a flare fastened to a parachute. Service engineers modified the Navy flare to fit the mortar shell, following the standard naval practice in mixing and loading flare compositions. They produced experimental shells that functioned perfectly but these came too late in the war for standardization.

In May 1944, the CG, South Pacific Area, asked for colored smoke mortar shells. Chemists developed mixtures containing dyes that produced red, yellow, green and violet smokes. With a fuze set for an air burst, shells containing these smokes erupted a cloud visible for several minutes in calm weather. Colored smoke shells were recommended for standardization shortly before the end of the war.

Starting with one basic type of shell in 1942, the CWS evolved a variety of shells for the 4.2" mortar. Only three of these, HE, WP and FS were employed in battle, but their effectiveness led Generalleutenant Oschner to say in speaking of the American mortar that, "the various types of ammunition used with it are excellent."

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Mortar Gunboats

The CWS, in 1942, experimented with mortars mounted on landing craft, including the LCIs and LCTs. It took the view that mortars could support an amphibious assault in the crucial period of an invasion, after the naval and air bombardment let up so that troops could land. Mortars could not be placed directly on the bottom of landing craft since there was no way to keep the recoil from kicking mortars backward when the piece was fired. In addition, the terrific pounding might damage the bottom of the vessel.

Technicians rigged an oblong wooden frame, filled with a mixture of sand and sawdust, on the floor of the craft. A thick slab of wood, grooved to take the bottom of the baseplate, sat on top of the sand-sawdust filling. The Amphibious Training Command, Carabelle, Florida, to which the CWS sent the firing platform, saw the utility of the device and asked the service to design a standard model.

In July 1943, a chemical mortar battalion [the 3rd] with weapons mounted in landing craft took part in the seaborne assault on Sicily. The battalion was ready to fire from its offshore positions but the need did not arise. Mortar gunboats first saw action in the Pacific where amphibious warfare was more common. On 15 Sep 1944, 4.2" mortars mounted on landing craft, infantry (LCI), supported the 1st Marine Div in its assault on Peleliu. Two days later, at Angaur Island, mortar gunboats fired on the beach area as troops of the 81st Div swarmed ashore. Offshore LCIs afterwards lobbed mortar shells into Angaur to support infantry attacks.

On D-Day at Iwo Jima, the heavy gunfire-support ships of the invading force were augmented by 18 LCIs armed with 4.2" mortars. During the night of Feb 20-21, LCIs with mortars delivered counter-battery and harassing fires in supporting the V Amphibious Corps. Since LCIs had no radar and only inadequate navigating gear, they followed the ingenious plan of steaming in an elliptical track around a reference ship that kept station by radar, firing during the time they were on the path of the ellipse and headed toward the island. On Easter Sunday of April 1945, 42 mortar gun-boats were among craft that led the way to the landing beach at Okinawa. 126 mortars laid down 28,000 shells over a strip 5½ miles long and 1000 feet deep in less than an hour. All in all, LCI(M)s (the M for mortar), as the mortar-mounted craft came to be designated, participated in a dozen landings in the Pacific during the latter part of 1944 and the war months of 1945.

Note: For a lot more about mortar gunboats, see Jack Butler's extensive writeup.

This excerpted history was published in serial form in the September 1996 and January 1997 issues of
The Red Dragon, the newsletter of the 2nd Chemical Mortar Battalion Association.

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