The Machine Gun

able. Because of the extremely large forces involved in transferring the required energy so rapidly from the barrel mass to the bolt, the parts of the accelerator should be ruggedly dimensioned and properly heat-treated for maximum strength and wearing qualities.

There is no ideal design type for an accelerator and many different types have been used with good results. Accelerators of various types arc illustrated in Part XI of this publication and therefore particular design configurations will not be treated in detail here. The present discussion is limited to the design factors affecting the use of acceleration in short recoil guns. The basic design requirements for an accelerating device can be enumerated as follows: The mechanism should, of course, be as simple as possible so that it can be manufactured with the least difficulty and is fundamentally reliable. The form of the mechanism should be such that its parts are compact and rugged and do not require delicate adjustment for effective functioning. If the foregoing requirements are satisfied, the next major consideration is one of "efficiency". In this connection, the term "efficiency" is used in a special sense. Since the object of using the accelerator is to speed up the bolt by making cfTcctive use of the kinetic energy in the recoiling barrel, the efficiency of the accelerator can be reckoned in terms of what percentage of the available energy is transferred to the bolt.

To analyze the factors involved in the transfer of energy from the barrel to the bolt, consider the type of accelerator shown in fig. 2-1 ft. (This type of accelerator is known as a "catapult spring" device.) With this type of accelerator, as the barrel recoils, the catapult strikes a latch in the breech housing and the barrel compresses the catapult spring as it moves to the rear in recoil. At the same time, the bolt moves to the rear until a lug on the bottom of the bolt is engaged by a catch on the top of the catapult. The catapult latch is then released so that the compressed catapult spring will drive the bolt to the rear. Without becoming concerned with the complications involved in properly timing the various latching and unlatching operations and other practical considerations, assume that all of the kinetic energy of the recoiling barrel is stored in the catapult spring (that is, the catapult spring brings the barrel to a complete stop) and further assume that all of this energy is transferred to the bolt without loss.

Now take the barrel weight as 45 pounds and the bolt weight as 5 pounds. Assume that the velocity of the barrel before starting to compress the catapult spring is 20 feet per second and that the velocity of the bolt just before the catapult is released is 32 feet per sccond (allowing for the increase due to blowback). The initial kinetic energy of the barrel (the amount of energy assumed to be stored in the spring) is:

The kinetic energy of the bolt before the catapult action starts is:

Since it is assumed that all of the kinetic energy originally possessed by the barrel is transferred through the spring to the bolt, the final kinetic en-crgv of the bolt will be:

KE3 = 280-{-79.0 = 359.5 (ft. lb.) The final velocity of the bolt will then be:

Thus it appears that even under the ideal condition of 100 per cent efficiency of energy transfer, the factor by which the bolt velocity is increased for the stipulated values of mass and velocity is only approximately 2 times. The factor will vary depending on the ratio between the barrel and bolt masses (increasing with this ratio) and will also vary slightly depending on the values of the initial velocities. Nevertheless, the values assumed in the example arc more or less representative of a typical 20-mm gun and it can be seen that there is a definite limit on the velocity gain that can be expected.

Under actual conditions, it is usually not practical to attempt to bring the barrel to a complete stop through the action of the accelerating device

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