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Mortars filled the gap between grenades and artillery. They were used to provide fire support when artillery either wasn’t available or couldn’t be moved up fast enough to support rapidly moving troop advances. As the Ordnance Chief remarked “Comparing weight of material to ammunition delivered on the target, mortars are the most efficient of weapons.”

Often called “The Infantryman’s Artillery”, the mortars of World War Two were basically improved models of the Great War mortars. They were used to provide short range, high-angle trajectory fire in direct support of the infantry.

The basic form of the mortar was designed by Sir Frederick Wilfrid Scott Stokes during World War One. The Stokes Mortar consisted of a smooth bore tube with a fixed firing pin at the bottom of the tube. The tube was fitted into a metal plate that rested on the ground and absorbed the recoil force. The front of the tube was supported by a bipod that was adjustable for elevation and traverse. The Stokes was not fitted with a sight, but was lined up on target by sighting along a white line drawn the length of the tube.

The Stokes weighed in at 110 pounds in the firing position and could be broken down into three loads for carrying: the tube (43 pounds), the bipod (37 pounds) and the base plate (30 pounds).

The mortar bomb used was a simple metal cylinder containing an explosive chare of two pounds of TNT. The entire shell weighed 12 pounds. The Stokes was fired by means of a shotgun-like cartridge that was inserted into the base of the shell and exploded upon impact with the fixed firing pin at the base of the tube. The range was determined by placing a number of small rings (made of silk bags) of explosive propellant (ballistae) around the cartridge container. The more ballistae rings used, the further the range.

With the end of the Great War, an evaluation of the combat use of the Stokes determined that there were two serious deficiencies: lack of range and accuracy, Both of these problems were due, in part, to the firing of a projectile without stabilizing fins from a smooth bore tube. The lack of an effective sight also had a negative bearing on the accuracy of the Stokes.

In the Great War Edgar William Brandt committed his French-based firm to the development of an improved Stokes mortar. Brandt recognized that the basic Stokes was a sound design and he concentrated on improving the unsatisfactory ammunition. He developed a mortar bomb with a streamlined shape and stabilizing fins that proved to be very accurate at ranges from 200 yards to 3,350 yards. In place of the silk bags of the Stokes shell, Brandt introduced celluloid packets or strips, which clipped to the bomb’s fins.

In 1927, the Stokes-Brandt Mortar, Model 1927 was introduced. This 81mm mortar was very similar to the Great War Stokes, but was provided with a collimator-style sight and a spring buffer to reduce the stress of firing on the bipod and sight. The Model 1927 continued to be refined and four were purchased by the United States for testing in 1931. These were designated the Stokes-Brandt Mortar, 81mm, T4. The T4 differed from the M1927 by having a improved bipod, sight and base plate.

While the T4 was undergoing firing trails, the Stokes mortar was redesignated as the Mortar, Trench, 3-inch, Mark I and Mark IA2. Several Stokes mortars saw service in the early days of World War Two (in the Philippines).

In the late 1930s, the United States purchased manufacturing rights from the Brandt Company and standardized the T4 mortar as the 81mm Mortar, M1 with Mount M1. The M1 weighed 136 pounds in its firing position and be broken down into three basic components for transport: the tube (44.5 pounds), the bipod/sight (46.5 pounds) and the base plate (45 pounds).

The M1 81mm mortar was very similar to the older 3-inch Stokes mortar, but had the following differences.
1) The tube was of heavier construction so as to better withstand the higher pressures generated by modern ammunition. The tube was also machined to finer tolerances than was the Stokes mortar.
2) A cross-leveling mechanism was attached to the left leg of the bipod.
3) The clamp that attached the bipod to the barrel was easily adjusted to four different positions.
4) The improved base plate consisted of a pressed steel body with welded braces, flanges and two carrying handles. A socket was welded onto the base plate which had three positions for the spherical end of the mortar tube’s base plate.
5) The M1 mortar was fitted with a greatly improved sight, which included a collimator, elevating and deflection mechanism and longitudinal and cross levels.

The M1’s range varied depending on the type of shell used, with a minimum range of 100-300 yards and a maximum range of 3,290 yards. The sustained rate of fire was 18 rounds per minute, but as many as 30-35 bombs per minute could be fired for short periods of time.

The 81mm mortar could be readily transported by three men, although it was normally transported in a jeep or weapons carrier in the infantry divisions. A number of half-tracks were converted to carry the 81mm mortar and was issued to the armored divisions.

81mm Mortar Ammunition
The M1 was a versatile mortar that could easily fire several types of ammunition.

M43A1 Light HE Bomb
This was the lightest of the 81mm mortar bombs, weighing 6.87 pounds and having a minimum range of 100 yards, and a maximum range of 3,290 yards. Eighty percent of its fragments covered a radius of about 25 yards, which compared favorably with the 75mm howitzer. The M43A1 was fitted with an fast detonating fuse so that the bomb would explode on the surface of the ground.

M45 and M45A1 Heavy HE Bomb
These were the heavier bombs used with the 81mm mortar, weighing 10.62 pounds. The maximum range was 2,558 yards. These bombs had a bursting radius comparable to that of the 105mm howitzer. These bombs were fitted with a delay fuse so that some penetration was possible for demolition use.

M56 Heavy HE Bomb
The heaviest of the 81mm mortar bombs, weighing in at 15.01 pounds. Its maximum range was only 1,300 yards. It was equipped with a fuse that could be adjusted for either super quick or delay operation.

M57 White Phosphorus Bomb
M57 FS Smoke Bomb
M57 HS Persistent Gas Bomb
Both types of smoke bombs weighed about 10.75 pounds and had a maximum range of 2,470 yards. They were intended to lay down covering smoke “in order to hinder enemy observation, either to reduce the effectiveness of hostile fire or to conceal the movements of friendly troops.” The WP bomb also had antipersonnel applications. The Gas bomb was loaded with a irritant tear gas filler, it was seldom used.

M301 Illuminating Bomb
This contained an illuminating compound that burned at 275,000 candlepower for about 60 seconds and had a range of 2,200 yards. The projectile was attached to a parachute which slowed its descent and increased its effectiveness. The M301 had a fuse that could be adjusted with a delay of 5 to 25 seconds after firing.

U.S. mortar bombs were issued assembled as complete rounds with the maximum number of powder increments attached. These were removed as necessary to achieve the desired range. While the system worked well, in general, problems were encountered with the exposed powder increments, particular in the humid climate of the Pacific. When the increments became damp, the range was severely reduced and rounds often fell short, with sometimes fatal results for friendly troops. The solution that was adopted during the war was to order the mortar crews to disposal of any increments that had been darkened by moisture.

The Quartermaster Unit of Fire issue for a single 81mm mortar was 275 rounds. This often proved to be very inadequate, with many reports of the entire daily quota of bombs being expended in as little as an hour. Stocks of 81mm mortar bombs could often run short and emergency resupply would be flown in by air transport. It is interesting that the Japanese and Germans used similar Stokes-Brandt 81mm mortars. There are numerous accounts of U.S. troops using captured enemy mortar bombs in their M1 mortars.

The usefulness of the M1 was simply stated in numerous Army and Marine reports, which referred to it as “the infantry’s artillery.”