WEAPONS OF THE WORLD WAR II MARINE

THE M1 81MM MORTAR

CHARACTERISTICS

The M1 81mm Mortar was a smoothbore, high-angle fire, muzzle loading weapon.

Weight: 136 pounds
Length of tube: 49.5 inches
Elevation: 40 to 80 degrees
Rate of fire (normal): 18 rpm
Rate of fire(maximum): 30 to 35 rpm
Range: 100 to 3,290 yards

The M1 81mm mortar was the largest weapon in the arsenal of the Marine infantry battalion. It provided the battalion commander with a powerful and flexible indirect fire capability. Four of these weapons were assigned to each battalion, in either the weapons or headquarters company, depending on the table of organization.

Sometimes called "infantry artillery," or "hip pocket artillery," mortars were capable of quickly laying down heavy barrages. These could stop enemy attacks under the worst conditions. Able to fire at high angles, mortars could fire at targets in defilade, either under direction of an forward observer, or firing off map coordinates. In the Pacific campaigns, these weapons became an important part of the battalion's firepower, especially since they could be man-packed into positions that were inaccessible to artillery.

During World War I, U. S. infantry battalions were equipped with the 3" Mk I Stokes Trench Mortar. This was the first truly man-portable mortar. With an effective range of 800 yards, the trench mortar could fire 6-10 rounds per minute at sustained rate, and 30 rounds per minute at rapid rate. After the war, the Mk I remained in service, although several abortive attempts were made in the 1920s to design a new mortar. Instead, the U. S. Army Ordnance Department finally settled on a program to develop better ammunition.

The M4 collimator sight as depicted in FM 23-90. This sight was also used on the M2 60mm mortar. By laying the sight on aiming stakes, the mortar squad could fire indirect missions. The open sight allowed the squad to fire direct lay missions. US Army Photo

French weapons designer Edgar Brandt used the Stokes Mortar as a starting point and engineered numerous improvements. Brandt's improvements made the mortar a much better weapon, with a sustained rate of 18 rounds per minute and a maximum range of 3,290 yards. The U. S. Army bought four prototypes in 1931 and refined the basic design. Later in the decade, the Army Ordnance Department purchased manufacturing rights to the mortar, designated the 81mm Mortar, M1, in U. S. service. Watervliet Arsenal served as the major producer of the weapons, but A. B. Farquhar Co., and Pullman-Standard Car Manufacturing also made the weapons.


The 81mm mortar squad as depicted in Table of Organization D-2, dated 28 March 1941. USMC Photo

As an infantry weapon, the M1 mortar could be broken down into three separate loads. These were the barrel, weighing 44.5 lbs, the base plate, weighing 45 lbs, and the bipod, weighing 46.5 lbs. The M4 collimator sight fitted into a bracket on the bipod yoke, providing accurate laying for elevation and deflection. Aiming stakes were supplied in the basic issue items for each mortar, enabling the crew to lay their weapon on target for indirect fire.

In the prewar era, the 81mm mortar platoon was located in the infantry battalion's headquarters company. In 1942, the D-series table of organization was introduced into the Fleet Marine Force. With this T/O, the battalion weapons company was introduced and the mortar platoon shifted to this unit for administrative purposes.   The platoon remained in the weapons company until mid-1944, when the F-series T/O was adopted. This change deleted the weapons company and the 81mm mortar platoon was again assigned administratively to the battalion headquarters company.

ORGANIZATION OF THE 81MM MORTAR PLATOON
 
Platoon Strength
Number of mortars
Mortar Squad strength
Company
D-series T/O
2+74
4
6
Weapons
E-series T/O
2+56
4
6
Weapons
F-series T/O
2+56
4
6
Headquarters
G-series T/O
2+56
4
6
Headquarters
Note 1: Platoon strength shows number of officers and number of enlisted Marines in two separate columns.

The 81mm mortar section as depicted in Table of Organization F-4, dated 27 March 1944. USMC Photo

Throughout the war and beyond, the four gun configuration remained standard for the platoon, which was split into two section of two tubes each. Two lieutenants were assigned to the platoon. One was the platoon leader and the other the assistant platoon leader. A gunnery sergeant served as the platoon sergeant and the sections were each led by a platoon sergeant. The mortar squad was the basic sub-unit within the 81mm mortar platoon, with a corporal as the squad leader. The following additional Marines were assigned to the squad: one gunner, one assistant gunner, and four or five ammunition bearers, dependent upon the current T/O.

MGySgt Lou Diamond
USMC Photo

MGySgt Lou Diamond was recognized throughout the Marine Corps for his outstanding mortar gunnery and there were countless sea stories about him. Early in the war he served as the platoon sergeant of the 81mm mortar platoon in 2nd Battalion, 5th Marines. In May 1940, the mortars deployed to Indiantown Gap, Penna., for live-fire training. Lou boasted that he was going to place a round into the chimney of an abandoned farmhouse in the impact area. After one adjusting round, he dropped the second round right on target.

One of the most famous stories about Lou and his mortar exploits happened during the Guadalcanal campaign. He was reputed to have fired several 81mm rounds at a Japanese destroyer, scoring a direct hit and driving the enemy ship off. In an official letter to Lou, the Commanding General of the 1st Marine Division, MajGen A. A. Vandegrift, wrote the following words, "As a master gunnery sergeant of Company "H", you performed your duties on Tulagi and Guadalcanal in an outstanding manner throughout the campaign. On several occasions, the well-directed and well-timed fire of the mortar platoon under your charge was a deciding factor in halting an enemy attack and enabling friendly troops to advance against enemy positions"

We have a weapon that nobody loves
They say that our gun's a disgrace
You crank up 200, and 200 more
And it lands in the very same place.
Oh, there's many a gunner who's blowing his top
Observers are all going mad.
But our affection has lasted
For this old pig-iron bastard
It's the best gun this world ever had.

The Mortarman's Lament
Anonymous

Several types of ammunition were issued for use with the 81mm mortar. All were fin-stabilized and came as complete units with fuzes already attached. The most common was the M43A1 high explosive round, with a weight of 7.05 lbs and containing 1.22 lbs. of TNT. Equipped with a superquick fuze, this round had a bursting radius of about 30 yards was commonly employed against enemy troops in the open. For dug-in or fortified enemy positions, the M56 high explosive round was available. Weighing 10.77 lbs, and with 4.30 lbs. of TNT, this round could be set for impact detonation, or for a short delay to enable it to penetrate before exploding. When used against troops in the open, it had a bursting radius of about 35-40 yards. Also available was the M57 chemical round. Filled with white phosphorous, this round was used for screening and obscuration and weighed 11.59 lbs.

During the fighting on Iwo Jima in 1945, a mortar squad engages in a fire mission. The gunner (at right) checks his sight to make sure it is aligned. The assistant gunner (at left) hangs an M57 smoke round in the tube. In the background, a stack of HE and smoke rounds is ready to fire. Sometimes, mortar platoons fired hundreds, or even thousands of rounds each day in combat. Still image from USMC combat camera film

An 81mm mortar squad of the 24th Marines places their weapon into service during training at Camp Pendleton, 1943. Crew drills were an essential part of mortar gunnery and Marines practiced them constantly. USMC Photo

REFERENCES:

The Organization of the Marine Infantry Regiment. Marine Corps Schools, Quantico. 1942.
The Organization of the Marine Infantry Regiment. Marine Corps Schools, Quantico. 1944.
FM 23-90 Basic Field Manual 81mm Mortar Gunnery. War Department. 1943.
U. S. Infantry Weapons of World War II. Bruce Canfield. Andrew Mowbray Publishers. 1998.
Ordnance Standard Catalog. War Department. 1945.
Leatherneck Magazine article "Diamond in the Rough," by Frank Tolbert. August 1943.

 

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