The Machine Gun

Although these advantages eliminated many of the shortcomings previously experienced with powder and ball, the use of metallic cartridge ammunition introduced the new problem of removing the spent cartridge case from the chamber after firing to permit the loading of a fresh cartridge. In repeating and semi-automatic weapons and in most machine guns, the problems of loading fresh cartridges into the gun and of extracting the fired cases from the chamber do not present any very great difficulties if the loading and extraction functions are given proper consideration in the design of the weapon. However, in guns having very high rates of fire and particularly in guns of the multiple chamber type, a special loading and extraction problem arises. It is not the mere process of inserting the loaded cartridge or of removing the cartridge case from the chamber that produces the problem but it is the small time that is allowable for the process to occur. For example, consider an automatic weapon which fires at a cyclic rate of 1500 rounds per minute. With a firing rate of this magnitude, only 0.04 second is allowable for the entire chain of events which must occur during each firing cycle. The particular series of eventĀ« which is necessary for the automatic functioning of the weapon will of course depend on the particular design characteristics of the mechanism but it can be seen that with such an extremely brief time interval available, the problems attendant upon any mechanical function can be greatly multiplied. For this reason, the means which are used to load the chambers of a multiple chamber weapon and to extract the fired cartridge cases must be given very careful consideration in the design. The consideration must cover, not only the mechanisms which accomplish the functions and the power sources used to drive the mechanisms, but must also take into account the timing of the functions, so that they can occur with no interference from the remainder of the moving parts of the weapon.

Another important factor in the design of a multiple chamber weapon is the control and dissipation of the recoil forces resulting from the firing of the cartridges. If the weapon is recoil-operated, a considerable portion of the recoil energy may be put to use in providing power for driving the gun mechanisms, but in a gas-operated gun, the forces of recoil do not contribute to the action and must be absorbed entirely by the mounting of the weapon. In a multiple chamber gun, the principles relating to recoil are the same as for any other gun. The ignition of the propellant charge produces an explosive combustion which causes the rapid generation of an extremely high gas pressure in the chamber of the gun. The expansion of these high-pressure gases drives the projectile forward through the bore of the gun and since the gases move down the bore after the projectile, the center of mass of the gases also moves forward. (Because of the fact that the gases are not moving forward as a single body but rather are expanding in the bore, the forward velocity of the center of mass of the gases is only approximately one-half of the projectile velocity.) As long as the projectile is still moving down the gun bore, the same pressure which acts to produce the forward motion of the projectile and powder gases also acts at the chamber end of the gun to produce an equal and opposite reaction which tends to drive the entire gun rearward. The force resulting from this reaction is called the "recoil force" and its magnitude at any instant depends on the chamber pressure which exists at that same instant.

After the projectile leaves the muzzle of the gun, the recoil force does not cease to act immediately but continues in existence as the reaction to the forward movement of the powder gases as they pass out of the gun muzzle. The recoil force does not actually fall to zero until the so-called residual powder gas pressure in the barrel has become equal to the atmospheric pressure. (In a representative 20-mm gun this docs not occur until approximately eight milliseconds after the ignition of the propellant charge.)

The combined effect of the recoil force which exists before the projectile leaves the muzzle and the recoil force produced by the residual pressure is to impart a rearward velocity to the gun and to create a kinetic energy in the gun mass. The magnitudes of the velocity and of the kinetic energy depend on the power of the cartridge, the mass of the parts subjected to the recoil action, and the manner in which the gun is mounted. In an analysis of the conditions of recoil, it is usually desirable to consider first what velocity and energy would be obtained if the gun is assumed to be mounted so that it is not subjected to retardation from friction or any other restraint. This hypothetical condition is referred to as the condition of "free recoil". Writh no restraint of any kind imposed upon the gun, the impulse of the recoil force

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