Recoil Operation

high speed air-to-air combat in which the gun can be brought to bear on target for usually only a split second, a rate of fire of 1000 rounds per minute might be considered none too high. In such combat, the target may be fairly light so that effective hits can be scored with ammunition of relatively small caliber. However, for targets such as buildings, ships, and heavily armored vehicles, it is usually desirable to use high-powered ammunition of large caliber and there probably will be ample time for firing. Under such conditions, a rate of fire of over 1000 rounds per minute is not necessary and it may be preferable to deliver a few devastating blows to the target rather than a large number of lighter hits.

Because of its advantages in regard to handling high recoil forces, the long recoil system is applied extensively for large-caliber weapons and, considering the caliber of these weapons, the rates of fire attained are fairly high. The disadvantage of the long recoil system in regard to rate of fire becomes evident only for smaller caliber guns. In such guns, the distances through which the recoiling parts must move are more or less predetermined by the total length of the cartridge used. Therefore, the only way in which the tendency toward a low cyclic rate can be minimized is to design the gun so that the recoil movement and the return movements of the barrel and bolt occur at the highest velocity attainable on a practical basis. Thus, in an analysis of long recoil as applied to smaller caliber guns, primary attention must be given to those factors which affect the barrel and bolt velocities.

Assuming the use of a particular type of ammunition, the main factor affecting the velocities of the recoiling parts is the weight of these parts. With a given ammunition, the total forward momentum imparted to the projectile and powder gases is some definite amount and the resulting reaction will produce an equal and opposite momentum in the recoiling parts. In other words, the momentum imparted to the recoiling parts is some definite amount which is determined by the cartridge used. The rccoil velocity which corresponds to this momentum will be inversely proportional to the weight of the recoiling parts and therefore the maximum rccoil velocity will depend on the weight of the recoiling parts. This necessitates that, to obtain a high recoil velocity, the recoiling parts should be made as light as possible.

Unfortunately, there arc definite limitations to how small the recoiling parts can be. In order to perform their functions and to withstand the forces to which they are subjected, the barrel, bolt, and other recoiling parts must be ruggedly constructed and will necessarily be fairly massive. In fact, all is based on the propellant charge and the more powerful the ammunition, the heavier these parts will be. Since there is a limit to how light the operating components can be, the maximum attainable rccoil velocity is sirnilarlv limited.

The extent of this limitation can be illustrated by considering a 20-mm gun. In a gun of this caliber with a maximum chamber pressure of 45.000 pounds per square inch, the barrel and barrel extension assembly alone could hardly weigh much less than 35 pounds, no matter how economically it is designed from the standpoint of weight. Allowing a conservative 6 or 8 pounds for the bolt, locking device and firing mechanism and another 6 or 8 pounds for the effect of the spring masses gives a total minimum weight close to 50 pounds. The recoil momentum produced by the assumed cartridge will be approximately 35 (lb. sec.) and dividing this figure by the mass of the 50-pound recoiling parts gives a maximum free recoil velocity of about 22.5 feet per second. Now it must be realized that this velocity represents (to a fair approximation) the highest value attainable in the assumed weapon bccausc an attempt to increase this velocity by lightening the parts would make the parts too frail.

Thus it appears that the designer of a 20-mm gun operated purely by long recoil is "stuck with" a maximum initial recoil velocity of somewhere near 22.5 feet per second. It will be recalled from the description of the long recoil cycle of operation that the barrel must recoil and counter-recoil the full distance before the bolt starts its return movement. For a 20-mm gun, the rccoil distance must be from eight inches to nearly one foot to permit feeding and therefore the barrel must travel a total distance of almost two feet per cycle. Even if it is assumed (although this is impossible) that this entire motion is accomplished at the maximum velocity of 22.5 feet per second and that the bolt return time is ignored completely, this will mean that the time required for each cycle will be 2/22.5 = .089 second which gives a rate of fire of 675 rounds per minute. Actually the barrel must be stopped and its motion

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