Among those not intimately connected with their manufacture or use there has long been a marked inability to differentiate between the various types of automatic guns. To the lay mind any self-operated weapon firing rifle ammunition is a machine gun and nothing else. It is quite natural that this erroneous impression should exist, as very little descriptive matter on this subject has been written except in official orders and in technical publications designed for the use of specialists.
The British classify automatic guns according to the manner in which they are fed, operated and cooled: An automatic rifle is magazine- or clip-fed, gas operated and air-cooled, while a machine gun is belt- fed, recoil-operated and water-cooled.
The War Department has designated as "automatic guns" all automatic firearms which fire a rifle cartridge, dividing them into five classes, viz.: Self-loading rifles, automatic rifles, machine rifles, aircraft machine guns and machine guns. The first and fourth types need not here be discussed, as no self-loading rifle has so far been adopted, and as aircraft machine guns are special types designed only for mounting on aircraft.
Automatic rifles are defined as automatic guns of such light weight and of such construction that habitually they are, or may be fired from the hip or shoulder in much the same manner as the magazine rifle. Types of automatic rifles are the Browning (light), Chauchat, Hotchkiss (light), and Berthier (with light barrel).
Machine rifles are automatic guns of an intermediate class between the automatic rifle and the machine gun. The cooling system of the machine rifle usually is more effective than that of the automatic rifle, and hence the machine rifle is capable of maintaining a sustained fire of longer duration than the automatic rifle. Its cooling system is not of great enough efficiency, how ever, to permit of long sustained fire at rates of 150 or more shots per minute without an undue increase in dispersion. The weight of the machine rifle, or its construction, or both, usually are such that habitually it cannot be, or is not, used as a hand arm as is the automatic rifle. Types of machine rifles are the Lewis, Colt, Benet-Mercier, Berthier (with special radiator), and Hotchkiss (heavy).
Machine guns are water-cooled automatic guns which, because of the efficiency of their cooling systems and the stability of their mounts, are capable of accurate and long-sustained fire at rates of 150 or more shots per minute, and this without any increase in dispersion that would endanger friendly troops over whose heads their fire may be directed. Types of machine guns are the Browning (heavy), Vickers, and Maxim.
The importance of distinction in the use of automatic guns can be no more accurately presented than in the language of W.D. Doc. 836, "Tactical Use of Machine Guns," A.E.F., France, 1918, where we find that "a clear understanding of the characteristics of machine guns is absolutely necessary to their correct tactical use. A confusion of ideas as to their powers and limitations mill lead to a failure to utilize them to the fullest extent, or to positive disaster by expecting from them functions which they are not capable of performing."
EMPLOYMENT
The automatic rifle is primarily a platoon weapon and is incorporated in infantry organizations.
The machine rifle is fired from a light mount and, while capable of firing for a longer period than the automatic rifle, has not the mobility and cannot keep up with the infantry. Like the automatic rifle, it is susceptible of use only in direct fire and can never be employed as a machine gun.
The machine gun is mounted on a tripod, weighing approximately 50 pounds. The legs are weighted with sand bags, so no matter how nervous the firer, the rigidity of the mount entirely eliminates the personal factor.
The fire of a machine gun is equal to that of 100 rifles, yet the space occupied is as one is to fifty. It depends for effect upon surprise and likes a deep, dense target.
The mobility of the machine gun depends greatly upon ammunition and water supply. Water is carried in breakers weighing (filled) about 12 pounds. It is essential that water be clean in order to prevent corrosion.
While the equipment of the machine gunners will not admit of keeping pace with infantry, it can be carried any distance and to any point reached by the infantry.
The machine gun is capable of rapidly producing and applying a large volume of concentrated and accurate fire. Because of its rigid mount it can employ the same types of fire as artillery. Nine-tenths of its fire is indirect, the most important type being overhead barrage fire.
Data is calculated for this fire in much the same manner as obtains in artillery, with the use of some adaptation of the mil formula, when a map is not available. Take the TOG method:
[(t x OT)-(g x OG)]/GT
When the target cannot be seen by the firer (No. 1) the machine gun officer, accompanied by the Range Finder Sergeant, takes up a position (O) where both a flank gun and the target can be easily seen, which position must be visible to No. 1. By means of the prismatic compass the slopes from to target (t) and gun (g) are taken, and the mil bearings recorded. The angle of site is the algebraic sum of (t x OT) — (g x OG). The angle of quadrant elevation is found by adding this angle to or subtracting it from the angle of tangent elevation (angle of departure) depending as the target is above or below the gun.
The ranges to target (OT) and gun (OG) are read, and the angle TOG plotted on a scale of about one inch to 100 yards. The triangle is then completed and the range scaled. The angle OGT is read, and No. 1 lays on O, turning off the angle found in the direction of the target. The elevation is then put on. Frontages of battery and target are paralleled and angles of concentration or distribution given.
Machine guns are always used in battery in this type of fire.
When one is available, the data is calculated from a map and recorded on charts similar to those appended. The most notable instance of the successful employment of indirect overhead barrage fire during the war was at the Battle of Messines, where 144 guns were used and nearly 3,000.000 rounds fired.