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4.2-inch Chemical Mortar
The Chemical Warfare Service adopted the M-1 4.2-inch Mortar in 1928. This rifled weapon could drop gas shells at an effective range of 2,000 yards. Limited numbers were procured and its manufacture was suspended in 1935.

With the entrance of the U.S. into the war, the War Department authorized the formation of two chemical mortar battalions. The original M1 was slightly redesigned to increase its effective range to 2,400 yards and was standardized as the M-1A1 chemical mortar. Further testing revealed that if the powder charge was increased by 50%, the mortar’s range could be extended to 3,200 yards. However, the tubes of the M1 and M1A1 could not safely handle the increased pressure and a stronger tube and base plate were adopted. This was designated as the M2. Both the M1A1 and M2 would see service throughout the war.

During the M2s development, it became apparent that the M2 was capable of more than firing gas and smoke shells. The CWS requested permission to develop a HE bomb. In spite of heavy opposition by infantry officers “who felt that the functions of the CWS was confined to gas masks, gas weapons and smelly clothing…”, the CWS got permission to develop the HE bomb. The testing of the new bomb so impressed the review board, that all opposition was overcome and permission to employ the chemical mortar with a HE bomb was forthcoming.

The 4.2-inch mortar first saw combat during the Sicily Campaign were its barrages with the new HE shell were impressive. The weapon saw use in the European and Pacific theaters and by war’s end, some 41,452 4.2-inch chemical mortars had been procured.

4.2-inch Chemical Mortar Ammunition
The chemical mortar used two types of HE shells during the war. The M3 HE Bomb weighed 24.5 pounds and the M4 HE bomb weighed 32 pounds. The various types of chemical and gas bombs weighed between 24.25 pounds and 25.5 pounds. These included WP and several types of irritant and screening gases.

While the 4.2-inch was useful in supporting the infantry, it was not as versatile as the 81mm and 60mm mortars. Ready for firing, the M2 weighed 333 pounds. Like the smaller mortars, the “four-deuce” could be broken down into three loads: tube (105 pounds); bipod and sight (53 pounds) and base plate (175 pounds)---each too heavy to be carried by one man for any distance. The 4.2-inch was most often carried by either a jeep-towed trailer or by a weapons carrier.

During the latter stages of the war, efforts were made to develop larger mortars. Two models, the 105mm T13 and the 155mm T25 saw what could only be described as "in the field test firing" during the Philippines Campaign. While they did deliver impressive amounts of firepower (the 105mm HE bomb weighed in at 26 pounds, with an effective range of 2000 yards. And the 155mm HE bomb weighed 60 pounds, with an effective range of 2500 yards), the sheer weight of the weapons preculuded their use. A review of records shows that the two models fired less than 3,000 rounds, before V-J Day.