World War II Indirect Fire Weapons
When asked to name indirect fire weapons used by infantry units, most people think of mortars. Yet, US Army infantry battalions in World War II fielded three indirect fire weapons. Each rifle company had two 60mm mortars, and the heavy weapons company employed six 81mm mortars. The third indirect fire weapon system surprises most people—the M1917A1 .30 cal. water-cooled heavy machine gun.
Wait! Aren’t machine guns direct fire weapons? Sure. The heavy machine guns served a crucial role in suppressing enemy weapons or mowing down enemy infantrymen with direct fire, especially in defensive situations. In offensive operations, machine guns had limitations on their direct fire role because advancing friendly infantrymen often masked their fire. The M1917A1 machine guns could overcome those limitations by using indirect fire techniques to shoot over the heads of friendly troops and pin down distant enemy positions.
What Is Indirect Fire?
What differentiates direct fire from indirect fire? In direct fire mode, a machine gunner uses a sight to aim at a target. He can adjust the sight to account for cross-wind and/or distance, but he still observes the target as he fires. In indirect fire mode, the gunner may not see the target. Instead he is given a direction of fire and a range to the target by an observer who has eyes on the enemy.
In order for an observer and gunner, working together, to place rounds on target, the weapon system must have 1) a mount calibrated for traverse and elevation and 2) a table of firing data.
The diagram below shows the machine gun and its mount. Just above the tripod, a traversing dial allows the gunner to swivel the direction of the gun left or right of center at a specified angle. The elevating arc sitting above the dial enables the gunner to raise the gun’s aim by a measured amount.
The army field manual (FM) for the machine guns printed firing data to tell the gunner how far a round would travel at a given elevation of the barrel--assuming the ground was level. For instance, to hit a target at a range of 1,500 yards the gunner had to set an “angle of elevation” of 35 mils, or about 2o. The FM also printed formulas to tell a gunner how to adjust for targets on higher ground and how much to elevate the gun to clear the heads of friendly troops moving forward.
The observer had the harder job. In a tactical situation, he had to know his own location and the location of the M1917A1s, then he had to spot an enemy position. The observer either had to identify the enemy’s grid coordinates on a map or visualize an imaginary line from from the guns to the target. With a compass, protractor and some math, the observer calculated the initial azimuth and range from the guns to the target. Once the machine guns fired tracer ammunition at the target, the gunner had to spot the strike of the rounds and estimate how far off the target they landed. He then applied a couple more formulas and called back corrections to the gunners until the rounds hit the target. Requiring all that math and estimation while under fire, placed a huge burden on the observers.
Infantry Plotting Board
The infantry observers’ lives got a whole lot simpler when the army issued plotting boards to the field in the summer of 1944. The plotting boards were fashioned with a clear plastic disk that rotated over a base with printed grid lines. The pivot point on the disk represented the position of the guns. The section sergeants, co-located with the guns, marked the observer’s known position on the disk using the grid lines on the base to correspond with map coordinates. When an observer called in an enemy target, he only had to tell the section sergeants the azimuth and estimated distance from his position to the target. Back at the gun position, the sergeants measured the angle and distance called in by the observer then marked the target position on the disk. The sergeant measured the line from the pivot point to the target position to determine the range and azimuth between the guns and the enemy. He gave that firing data to the gunners, so they could set proper direction of fire and elevation on the guns. After the guns fired spotting rounds, the observer only had to call back corrections as he saw it from his position. The sergeants with the plotting boards issued the corrected directions of fire and ranges to the gunners.
The heavy machine guns played an important role in the attack because they could employ indirect fire techniques. With the advent of the simple plotting boards the infantry battalion could get indirect fire from the mortars and heavy machine guns onto enemy targets more accurately and faster than before. You can read more about how World War II infantry units made use of the plotting boards in my book Battle Hardened.