The 4.2 Chemical Mortar
from the Infantry Journal, March 1944
by Brigadier General Alden H. Waitt
A few weeks ago a Washington newspaper carried the announcement of a new sensationally successful "secret" weapon – the 4.2 inch chemical mortar. Those of us who have watched and shared in the development of this mortar for the past twenty years, and have been "telling the world about it" were pleased that the secret was out at last. It appears that the advantages of the chemical mortar are rapidly being realized under actual battle conditions. This "Cinderella" weapon is today being accepted enthusiastically.
The Infantry have seen it in action and have found that they need it to do their job. The chemical mortar covers that range between the artillery and the smaller infantry mortars, delivers a powerful blow and with its high angle fire reaches defiladed spots which the flat trajectory guns and even the howitzers of the artillery are unable to reach.
While in the North Africa and Italy last October, I talked with many officers at Allied Force Headquarters and on the staff of the Fifth and Seventh Armies. Everyone spoke with enthusiasm of the 4.2 mortar, of the terrific morale and casualty producing effect of its phosphorus shell and the devastating punch of its high explosive shell. There is no doubt in anyone's mind that the weapon has proved itself. All agreed that the phosphorus munition is sensational in its destructive effect on the enemy.
The 4.2 chemical mortar developed out of the experience of trench warfare in the First World War. Because artillery weapons could not reach men in trenches, it was necessary to devise an all-around high angle fire weapon that would reach them. The answer was found in the Stokes mortar. Later in the war the Stokes was adapted by the British to meet the needs of gas warfare, which required a mobile gun, that would fire rapidly, and deliver high concentrations of gas on small targets. It was accurate enough for gas or smoke uses and within its very limited range of 200 to 1200 yards was effective against battalion targets. It was a special purpose weapon but not versatile enough to hold its place in the peacetime army.
Following World War 1, work was carried out for many years to develop a mortar, which would retain all the good features of the four-inch Stokes mortar and would add increased range and greater accuracy. It was known that if the awkward tumbling flight of the Stokes shell could be straightened out that it would bore through the air like a rifle bullet, the range could be more than doubled. The 4.2 inch chemical mortar, now the principal weapon of chemical troops, is the result of this development.
It is muzzle-loading, rifled, and fires a high-capacity shell at a high angle with the accuracy of an artillery piece at ranges from 600 to over 4000 yards. For sustained fire it can deliver five rounds per minute for an indefinite period. For short periods a rate of twenty to thirty rounds per minute can be obtained by trained crews. It is mobile and can be moved forward on its two-wheeled carts at the same rate as an infantry advance. It has a low silhouette (three and one half feet high when installed) and can be concealed easily. It can be fired from small gulches, shell holes, or from behind steep ridges or buildings. Within its range it is particularly useful for the support of attacking troops either with gas, or high explosive. The desired range is obtained by varying the elevation of the piece and the amount of propellant.
The mortar consists of a barrel, a standard, and a base plate. Its equipment includes certain spare parts and accessories for its installation and maintenance, and a two-wheeled, rubber-tired, hand-drawn cart for transportation in forward areas. The barrel complete weighs ninety-one pounds. The standard weighs fifty-three pounds. The base plate is the heaviest part of the mortar, weighing 155 pounds.
The mortar shell has thin walls and large capacity, and was designed especially for firing chemical agents. It weighs approximately twenty-five and one-half pounds ready to fire and holds six to eight pounds of chemical. It is prepared for firing by inserting a cartridge into its base and placing the cartridge container rings of powder sufficient to give the desired range. When the shell is loaded into the muzzle of the mortar it slides down to the bottom of the barrel where the cartridge is ignited by the striker pin. The cartridge then ignites the rings. The explosion expands the soft metal plate at the base of the shell so that the shell engages in the rifling of the barrel, thereby giving the shell a rotating flight. Inside the shell is a perforated steel vane, which causes the liquid filling to rotate with the shell and give the shell added stability in flight.
The 4.2 mortar has been used with success by mechanized troops. Installed on a mechanical mount, it has the same mobility as a tank, personnel carrier, or other track-laying vehicle. It should be especially useful to armored forces laying smoke screens to permit advance against anti-tank gunfire, or to conceal movement to attack positions.
The chemical mortar is the most effective mobile weapon available for firing gas or smoke within its ranges. A chemical company firing at the maximum rate can place nearly two tons of a non-persistent chemical agent such as phosgene on the target in two minutes. This is enough gas to put completely out of action every unprotected man in an area of a quarter of a million square yards. Should the requirement be to lay down mustard gas, a chemical company can fire six tons in thirty minutes, an amount sufficient to produce an effective concentration on about a million and a quarter square yards. A platoon under average conditions can maintain an effective smoke screen across a front of over 1000 yards.
One platoon in Sicily covered a hill crest with WP smoke nearly one whole day lifting the screen occasionally to permit dive-bombing attack. A single white-phosphorus mortar shell, when it bursts, covers an area of about forty yards in diameter and will set fire to easily combustible substances or cause casualties. No other weapon as yet developed can even approach the 4.2 mortar in all around effectiveness in firing smoke and gas. It offers tremendous possibilities in open and jungle warfare with its recently standardized high explosive shell.
New tables of organization for the motorized chemical battalion call for a highly mobile and effective unit. The new chemical mortar squad has a sergeant squad leader, a corporal and four privates. This squad is undermanned and according to reports from the field and must be increased. The personnel of the squad, the mortar, the carts, the ammunition, and equipment are transported in two ¼ ton trucks (jeeps) and ¼ ton trailers. The first jeep carries the squad leader and three men (the mortar crew) and its trailer carries the mortar, its cart, and eight rounds. The second jeep carries two men (ammunition handlers) with eight rounds, and its trailer carries the ammunition cart and about twenty-four rounds. An increase in personnel, which I believe is necessary since four men are needed to pull the loaded ammunition cart, will reduce the number of rounds that can be carried in the jeep and the trailer. However, other vehicles are included in the organization to bring up additional ammunition and when unloaded the jeep and trailer can be used for shuttling. The cross-country ability of the jeep makes it possible to transport the company practically anywhere troops can operate. By firing smoke or high explosive, the 4.2 units can furnish invaluable support to infantry units in a tight place.
For amphibious operations the mortar squad can easily be carried with jeep and trailer on landing boats so that it is available for instant use in covering the infantry advance to secure the beachhead. The weapon may be mounted on the landing craft and support the actual landing with smoke. Smoke shell will detonate on the surface of the water and provide an effective smoke screen to permit approach to a defended beach. The mortar in such cases is aimed by steering the boat. As soon as boats are in mortar range of the beach, WP shell may be fired for combined smoke and casualty affect, of HE may be used to destroy enemy field works or to cause casualties.
Chemical weapons battalions armed with the mortar went into action for the first time in Sicily. They fired WP and HE. Today they are in constant use in Italy. Recognition of the mortar has been slow because it has been essentially a gas and smoke weapon. Gas has not yet been used and the value of smoke until recently has not been proven in combat.
The adoption of a high explosive shell, however, and the recognition of the casualty effect of phosphorus have increased its versatility and given it every day usefulness to the infantry. The new battalions have been indoctrinated in one major purpose: close support to the infantry.
Recent reports released on the Sicilian campaign devote much space to the employment of the 4.2 chemical mortar and its accomplishment in battle. Infantry officers whose units were supported by chemical troops give enthusiastic testimony to the effectiveness of their support.
I have abbreviated these comments and deleted those portions which deal only with organization and the requirement for a new method of transportation that has been taken care of by an additional jeep.
The chemical mortar is simply grand. In this battalion we are completely sold on it. We think the attached chemical company did a marvelous job. These mortars proved tremendously effective for all sorts of missions, especially against machine-gun nests, strong pockets of resistance, prepared strong points, pillboxes, and targets in defiladed positions beyond the range of our regular mortars. They are accurate as the devil and pack a punch worth two 81 mms. We dropped one round behind the corner of a house and killed nine Germans who were taking cover behind it. I don't know how many more were wounded by this one round. We think the mortars are the finest weapons we have seen. A German prisoner referred to them as "automatic artillery". You can fire them at terrific speed – just as fast as you can drop the bombs in the tube.
Lieutenant Colonel, Infantry:
The mortar is most effective with white phosphorus and HE. The Germans are very allergic to white phosphorus anyway, and we would root them out of their holes with well-placed rounds of phosphorus, and when we had them above ground, we plastered them with HE. We killed large numbers of them that way, and they dreaded the mortars. The 4.2 is equivalent to real artillery. We also used them in the assault coming ashore. I have said we used them, I really mean the supporting chemical troops used them. They did such a good job with us we got to regard them as our own people. The prize package was one day when a chemical mortar officer actually dropped a round of HE from one of his mortars into an open turret of a German tank.
Lieutenant Colonel, Infantry:
The 4.2 chemical mortars are marvelous weapons. After we had a platoon attached to the battalion, I was scared to death they would take them away from us and attach them to some another outfit. The demand for them was so great, The Germans were deadly afraid of them and the prisoners told us they thought they were some kind of new secret weapon like an automatic cannon because they had such tremendous effect and could be used so rapidly. Without exaggeration I would say that the 4.2 is the most effective weapon used in support of the infantry that I have ever seen. We have yet to see an enemy position that was tenable when we fired on it with WP and HE from this mortar. They can reach into almost perfectly defiladed positions and their effect is devastating. A great advantage lies also in the fact that the projectile is silent, and still you have the effect of a 105mm. shell burst, with greater rapidity of fire. Another advantage is that if the mortar comes under artillery fire, you can take it up and move it to a new position at once. And it is as easy as pie to camouflage and conceal. I have seen these mortars used to mark base and reference points for the artillery using WP. They also have tremendous range for an infantry mortar, and are effective and accurate.
Lieutenant Colonel, Infantry:
The Germans hate and fear white phosphorus, especially when you deal it out to them with the 4.2 chemical mortar. We had a platoon of these mortars attached to us and they were remarkably effective. They were used for all kinds of missions, including night harassing and interdiction fire close behind the enemy lines like real artillery. We would also use them with phosphorus to interdict the lines of escape when we had the Heinies on the run to keep them from getting away. The 4.2 was one of the most powerful and effective weapons we had with us. The chemical troops did a grand job.
The 4.2 mortar? I think that it is one of the finest weapons in existence. The effect on the Germans was tremendous. They feared them more than anything else, except our artillery barrages. Especially the Germans hated the WP the chemical troops rained on them. One German prisoner who spoke English is reported to have asked, "What the hell was that new weapon that had the effect of medium artillery that blew us out of our foxholes no matter where we were and burned our tails off at the same time?" Other prisoners thought that it was another new secret cannon that fired like an automatic ack-ack gun. The ammunition problem is great. But carts of chemical troops work well when hooked to a jeep. They hooked several ammunition carts in tandem, and this worked well. The jeep should be made part of the organic mortar troops equipment. Give them enough jeeps to haul the ammunition and to get the mortars around, and they'll keep them in action anywhere.
White phosphorus with the 4.2 chemical mortar gave remarkable results. It was used with tremendous effect, particularly against pillboxes and strongpoints, prepared positions, etc. Letters captured and taken from prisoners have shown that the Germans fear WP more than anything else, and from these mortars, our fire power with this projectile was tremendous.
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