Classic Anti-Aircraft Sights
By Robert G. Segel
For every offense there is a defense. So it was when the first aircraft were outfitted with machine guns and bombs that an appropriate defense had to be formulated to address this new threat. A fast moving weapons platform operating in a three dimensional space required a new set of skills to defeat. Since machine guns are capable of firing a high volume of bullets in a short period of time, they were the logical choice as the weapon to shoot down bombing and strafing aircraft. Though anti-aircraft fire is a form of direct fire, special considerations had to be taken in to account to properly lead an aircraft and learn the use of tracers to help determine the bullet stream.
Following is an excerpt for marksmanship for air targets as written in the Basic Field Manual FM 23-55 for the Browning Machine Gun, Caliber .30, M1917, 1940 edition. While the principles hold true whatever the infantry machine gun being used, in World War I tracers were not generally available. The only advantage the gunner had was that those planes flew low and slow.
MARKSMANSHIP FOR AIR TARGETS ANTI-AIRCRAFT GUNNERY
134. Types of Combat Targets. - All hostile airplanes which fly within 1,000 yards of the gun position are considered to be suitable targets. Such aircraft will present two distinct types of targets to the gunner, either type of which may be flying at a constant altitude, or decreasing or gaining in altitude.
a. Overhead. - Those that pass directly over the gun.
b. Nonoverhead. - Those that do not pass directly over the gun.
136. Observation of Fire. - a. Machine gun anti-aircraft fire generally employs individual tracer control. In this method the gunner controls his own firing as the result of his own observation of the tracers.
b. The gunner can observe his own firing in one or two ways.
(1) By observing the tracers in the vicinity of the target. In this method the gunner is conscious of the entire tracer stream which seems to curve sharply into the target. Considerable training is necessary to adjust fire by this method as an optical illusion must be taken into account.
(2) By observing the tracers close to the gun. Gunners using this method observe the first part of the trajectory of the tracer bullets which appear to them as a straight and bright white streak. This method of observation is prescribed in this manual for troops whose training is primarily with ground machine guns.
c. While the length of the white tracer streak is about 90 yards, the gunner is not conscious of this length but sees the streak as extending out towards the target. The eye position of the gunner can vary from about 2 inches above the receiver to 8 or 9 inches above the receiver, without materially affecting the accuracy of fire. When using the white tracer streak, the gunner is in effect using a line extending the axis of the bore of the weapon. By means of this streak he can manipulate the axis of the bore of his weapon to set off the required lateral lead and visual elevation. If the gunner sees the tracer as red, he is observing the farther part of the trajectory and is not applying the white tracer streak method of tracer control.
137. Visual Elevation. - Visual elevation is simply the distance above the target that the white tracer streak must appear in order to compensate for the drop in the trajectory. This visual elevation is expressed in target heights. Table I shows the amount of visual elevation in target heights required for a plane with a height of 6 feet.
138. Leads. - a. In order to hit a rapidly moving nonoverhead target it is necessary to direct the axis of the bore of the weapon an appropriate distance ahead of the target so that the bullet and target will meet in space. The distance of the aim ahead of the target is called the lead. It is expressed in target lengths.
b. The following lead table shows the target length leads for firing at planes out to 700 yards in range.
139. Techniques of Fire. - a. General - (1) A plane traveling at 200 m.p.h. will cover a distance of 1,000 yards in about 10 seconds. Every second of the 10 seconds available should be utilized for firing.
(2) Planes whose course will not take them within 1,000 yards of the gun are not generally engaged as a lead of unusable length would be required. Every plane whose course will take it within 1,000 yards of the gun position must be engaged for the entire time it is within that range.
b. Nonoverhead planes. - The gunner will open fire at range of 1,000 yards with a lead of 9 target lengths to permit him to settle into his firing. The gunner tracks the plane in its direction of flight and quickly establishes a lateral lead of 9 target lengths and visual elevation of 1 target height. As the range to the plane decreases, the gunner reduces the lateral lead and visual elevation, until the plane is at its closest range. Lateral leads and visual elevation are then increased to correspond to the increasing range of the plane. If the plane appears at a lesser range than 1,000 yards, fire is immediately opened with a lead based on the estimated range and the process is the same.
c. Overhead planes. - the variable lead method of firing as described in b above, does not apply to overhead planes. Such targets are engaged in accordance with the following simple rules:
(1) Rule one. - Incoming targets - the tracers must appear to pass just over the nose of the target.
(2) Rule two. - Departing targets - the tracers must appear to pass just under the target.
140. Estimation of Ranges. - Gunners should be trained to estimate the approximate range to hostile planes. Whenever possible, such training should be given by having an airplane fly at known ranges until all are familiar with the appearance of the plane at key ranges. In any case table III will be found useful.
141. Ammunition Loading for Anti-Aircraft Firing. - All tracer ammunition is the most desirable loading for anti-aircraft firing with infantry machine guns. However, ammunition loaded in proportions of 1 tracer and 1 ball up to 1 tracer and 3 ball gives effective results. The white tracer streak is apparent with these mixed loadings. When the proportion of tracer to ball is below 1 to 3 rounds the effectiveness of the fire is reduced. Loadings for the field are in accordance with the tactical and supply situation. Loadings for training purposes are in accordance with available ammunition allowances.
142. All Ball Ammunition. - The use of all ball ammunition, while giving somewhat lower hit percentages than the tracer control method, is practical and desirable for use by troops in the field in circumstances where the tracer loadings are not readily available.
143. Firing With All Ball Ammunition Loading. - The following method of firing which employs all ball ammunition loading is suitable for anti-aircraft firing. The method of firing is generally the same as for tracer control. The eyes look over the receiver and water jacket. Lateral leads are established by sighting over the rear sights and end cap to lay the axis of the bore the desired lead ahead of the target. Visual elevation is obtained by laying the end cap the desired height above the line of flight of the target.
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