Residual Pressure

dition known as "retarded recoil". In many guns, the effect of recoil is merely an inconvenience which must be tolerated since it can not be avoided. The primary source of difficulty in such cases is the tremendous magnitude of the recoil forces resulting from firing high-powered ammunition. (For example, it might be pointed out here as a matter of general interest that maximum recoil force in a 16-inch gun having a maximum chamber pressure of 38,000 pounds per square inch is the stupendous figure of almost 8,000,000 pounds. Even in a 20-mm gun, the maximum recoil force can amount to over 22,000 pounds.) While it might be possible to build a rigid mount capable of directly withstanding forces of this magnitude, such a mount would be entirely too heavy and cumbersome for any practical purpose. Accordingly, it is neccssary to permit the gun to move during and after the action of the recoil force so that the momentum of the gun can be cancclled through the application of a smaller retarding force which acts over a considerable, interval of time and distance. Since the gun moves in recoil, it acquires kinetic energy which must be absorbed in order to bring the gun to rest. For the above reasons, many guns (particularly those of heavier calibers) are mounted in a slide and arc provided with a recoil brake and a recuperator, both of which absorb the recoil energy. Most of the energy is dissipated by the brake in the form of heat and the energy stored in the recuperator is utilized to return the gun to battery.

In hand-held weapons, the recoil momentum is cancclled by a force applied to the weapon through the arm or shoulder of the firer. For example, when a Springfield rifle is fired, the maximum force of recoil is about 3700 pounds. (The maximum chamber pressure of 52,000 psi times the bore cross-section area {ยป/4) (.30)" in., gives the maximum force as 52,000X .0707=3670 pounds.) Actually, this force is not applied directly to the shoulder of the firer but produces the rearward acceleration of the gun known as "kick." The shoulder of the firer then acts as a recoil brake which absorbs the resulting kinetic energy of recoil over a travel of two or three inches. If it is assumed that the rifle moves three inches in recoil, the average force applied by the shoulder against the stock must be in the order of 60 pounds, if an attempt were made to limit the recoil distance to one-quarter inch, the average force would be 720 pounds. (This indicates why it is not a good idea to fire a high-powered ritte with the shoulder rigidly backed up against a solid support.)

Up to this point, recoil has been considered as undesirable because it crcates forces and energies which must be absorbed and dissipated after each shot of an ordinary gun. However, in an automatic weapon, these same forces and energies represent a convenient source of power for operating the gun mechanism. Machine guns which utilize this source of power are said to employ the "recoil system" of operation.

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