Meet America’s ‘Goon Gun’: The M2 4.2‑Inch Mortar That Won World War II

 In Germany, GDI, Land, Defense, Air

Key point: While the mortar units were a valu­able asset, they were not with­out their prob­lems.

In the last days of March 1945, a sol­dier named Carl Getzel sat on a hill out­side the city of Aschaffenburg and watched as it was slowly destroyed. The young pri­vate was part of the American 157th Infantry Regiment, 45th Division. His unit was tasked to take this town but it had been hard going; the German troops inside the city were putting up a deter­mined defense.

At ech­e­lons far above Carl, the deci­sion was made to bom­bard the city. Artillery and air strikes pound­ed the once-beau­ti­ful town, but from his van­tage he saw the a third, little known weapon wreak its own havoc. Not far behind him sat Company C, 2nd Chemical Mortar Battalion, equipped with the M2 4.2‑inch mortar. Bombs from the mor­tars flew over­head to land in the city. As he watched, the pro­jec­tiles crashed into build­ings, bring­ing sev­er­al down. It was an awe­some dis­play of fire­pow­er; Company C would fire 1,519 rounds of ammu­ni­tion into Aschaffenburg during the last three days of March alone.
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The Making of the Chemical Mortar

Although this account describes the 4.2‑inch mortar (com­mon­ly nick­named the “four-deuce”) as a close sup­port weapon for front­line troops, the weapon was orig­i­nal­ly con­ceived for deliv­er­ing poison gas and smoke muni­tions. Development of the M2 began after World War I as an attempt to improve the British-designed Stokes-Brandt 4‑inch mortar. Since the intent was to uti­lize the new mortar solely for chem­i­cal weapons (smoke being con­sid­ered a chem­i­cal weapon of sorts), the weapon came under the con­trol of the U.S. Army Chemical Warfare Service. That branch wanted a weapon that had a longer range and was more mobile than the Stokes design. In addi­tion, it wanted a weapon that had the normal advan­tages of a mortar: high rate of fire, the abil­i­ty to deliv­er fire­pow­er quick­ly and to go into action in a short time.

Mortars are small­er and lighter than cannon of com­pa­ra­ble cal­iber and fire at a high angle, their rounds arcing high into the air and coming down at a nearly ver­ti­cal angle to the ground. This enables mor­tars to fire at tar­gets in defilade, hidden behind objects such as build­ings or hills that would mask artillery fire. Mortar ammu­ni­tion, tech­ni­cal­ly called “bombs,” is not sub­ject­ed to the same pres­sures as artillery shells so it can be made with thin­ner walls and of cheap­er metals, enabling it to carry higher pay­loads of explo­sives or chem­i­cals. The pri­ma­ry dis­ad­van­tage of a mortar is its rel­a­tive­ly short range.

Initial exper­i­ments con­duct­ed after World War I used left­over Stokes weapons and muni­tions, but these trials result­ed in the improved 4.2‑inch model’s appear­ance by 1924, under the tute­lage of Captain Lewis McBride. Like many of the weapons des­tined to play a part in World War II, the M2 suf­fered from the cur­tailed defense bud­gets of the 1920s and 1930s. Only a few of the mor­tars were pro­duced, and they remained large­ly exper­i­men­tal with trials con­tin­u­ing essen­tial­ly up to the war’s begin­ning.

One set of exper­i­ments begin­ning in 1934 involved cre­at­ing a high-explo­sive round despite the M2 being ear­marked as a chem­i­cal weapon deliv­ery system. While no one at the time wanted to give up the M2’s pri­ma­ry role of launch­ing gas and smoke, it would prove a for­tu­itous deci­sion, making it a true dual-pur­pose weapon when war arrived years later.

Despite this exten­sive test­ing and plan­ning, the four-deuce almost did not sur­vive to see combat. In 1935, the War Department stopped all pro­duc­tion of the weapon and later des­ig­nat­ed

the small­er 81mm mortar as stan­dard issue for chem­i­cal bat­tal­ions. The 81mm was an excel­lent light weapon for the infantry, but for the chem­i­cal branch it was con­sid­ered much less capa­ble. It was not until war clouds were loom­ing close that the 4.2‑inch was res­ur­rect­ed after a meet­ing between General George C. Marshall, Army chief of staff, and the com­man­der of the Chemical Branch.

The M2’s poten­tial as a close sup­port weapon was obvi­ous, and the chem­i­cal branch imme­di­ate­ly began cam­paign­ing to adopt the high-explo­sive ammu­ni­tion; while smoke muni­tions were of obvi­ous use, the war thus far had seen no use of poison gas. It would be a waste of resources to rel­e­gate the mortar to screen­ing mis­sions only, and sol­diers in the Chemical Warfare Service (CWS) saw it as a way to increase the role their branch would play in the war.

This would not be the last bureau­crat­ic stum­bling block for the M2. When the CWS pre­sent­ed the case for the mortar to the Army ground forces, it rec­om­mend­ed the Field Artillery Board test the weapon first. Should the artillery approve the M2, it would be used as a sub­sti­tute for the 105mm how­itzer in the­aters where that weapon was not in use.

Since there were few places the 105 did not go, it would be the death knell for the mortar. The ser­vices of Supply Branch sup­port­ed the CWS, citing the mortar’s higher fire­pow­er, lighter weight, and lower man­pow­er require­ments for crews as rea­sons to adopt it along­side cannon. In the end, oppo­si­tion simply died out, and high-explo­sive ammu­ni­tion for the M2 was approved on March 19, 1943. The four-deuce was going to war.

The mortar that had been the center of such con­tro­ver­sy had a cal­iber of 4.2 inches or 107mm. It was a con­ven­tion­al design in that it was muzzle loaded; the crew dropped rounds into the tube from the muzzle. The bomb would slide down the barrel, and its end would strike a firing pin that would det­o­nate the pro­pel­lant, send­ing the round back up the tube and on its way.

Unlike most mor­tars, the M2 had a rifled barrel to impart a spin to its pro­jec­tile, increas­ing accu­ra­cy. The weapon was com­posed of three parts — the 175 pound base­plate that sat on the ground and absorbed the recoil of firing, a barrel 40.1 inches long, and the stan­dard. This piece is con­nect­ed to the base­plate by two con­nect­ing rods and has a single ver­ti­cal piece that houses the ele­vat­ing mech­a­nism. The mortar is also equipped with an opti­cal sight that lines up on aiming stakes sim­i­lar to artillery. The entire assem­bly weighs 330 pounds.

The weapon could tra­verse seven degrees to the left or right, and ele­va­tion was between 45 and 60 degrees. Up to 20 rounds per minute could be fired for short peri­ods, though this was often exceed­ed in combat. Minimum range was 600 yards. Maximum range, ini­tial­ly just over 2,000 yards, was improved by war’s end to 4,500 yards, again com­mon­ly exceed­ed in action. High-explo­sive bombs weighed in at about 25 pounds, while smoke rounds were heav­ier at 32 pounds. In prac­tice the weapons were often called “chem­i­cal mor­tars” since they were only used by the CWS.

The early chem­i­cal mortar units varied in size from com­pa­ny to reg­i­ment. Standardization was soon intro­duced with the bat­tal­ion as the stan­dard unit, though a few sep­a­rate com­pa­nies did see ser­vice in the Pacific the­ater. Battalions had four com­pa­nies, each with two pla­toons. A pla­toon was fur­ther divid­ed into two sec­tions, each with three mor­tars, total­ing 12 weapons per com­pa­ny and 48 per bat­tal­ion. By mid-1942, six bat­tal­ions exist­ed: the 2nd, 3rd, and 81st through 84th. Four more were cre­at­ed by mid-1943, the 85th through 88th.

Combat Debut in Italy

The bap­tism of fire for the M2 came with the inva­sion of Sicily, Operation Husky. Four bat­tal­ions — 2nd, 3rd, 83rd, and 84th — were chosen to go ashore. There were two weak­ness­es. Only two com­pa­nies of the 3rd Battalion had received amphibi­ous train­ing, and only the 2nd had con­duct­ed train­ing along­side infantry units. Time lim­i­ta­tions before the land­ing meant little work could be done to cor­rect the prob­lems.

Nevertheless, the mortar units went ashore and quick­ly proved their worth. When the Rangers fought at Gela, the 83rd sup­port­ed them, help­ing repulse enemy tank and infantry attacks. The 2nd Battalion fin­ished off a dis­abled German armored vehi­cle that was firing on American troops, firing eight rounds that all landed in an area only 15 yards in diam­e­ter with one round going right into the open top of the vehi­cle. The mor­tar­men also laid large smoke­screens to screen friend­ly move­ments, in one case keep­ing the screen up for nearly 14 hours. Unit com­man­ders were gen­er­al­ly very pos­i­tive about the effec­tive­ness of the chem­i­cal mortar sup­port, smoke, and high explo­sive, one even call­ing it “the most effec­tive single weapon used in sup­port of infantry.”

Still, as with any weapon new to combat use, some short­com­ings were noted. Users wanted a longer range. At this point the M5A1 pro­pel­lant gave a max­i­mum range of 3,200 yards (the 4,500-yard M6 pro­pel­lant was not yet avail­able). The sight was also crit­i­cized as having too small a tra­verse and lack­ing illu­mi­na­tion for night firing. Using typ­i­cal G.I. inge­nu­ity, Americans adapt­ed cap­tured Italian 81mm mortar sights, which proved to be a better piece of equip­ment.

After Sicily, chem­i­cal mortar bat­tal­ions went on to serve at SalernoAnzio, and through­out the Italian cam­paign. One bat­tal­ion, the 83rd, became known as the “Artillery of the Rangers,” begin­ning its rela­tion­ship with Colonel William Darby’s Ranger force at Gela on Sicily. The Rangers had no organ­ic heavy weapons, and the M2’s fire­pow­er was a wel­come addi­tion. During the first two weeks of the Salerno cam­paign, C and D Companies alone fired more than 14,000 rounds. When the Rangers seized Chuinzi Pass and Mount Saint Angelo, they faced heavy coun­ter­at­tacks from German forces. The fight­ing became so des­per­ate that mor­tar­men had to take their place on the line as infantry. Many tar­gets were engaged up to 2,000 yards beyond the four-deuce’s range. The strain of such hard usage caused large num­bers of break­ages in mortar parts.

Source: National Interest

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