## The Machine Gun

This increase in velocity is indicated by sloping the velocity curve after locking occurs by an amount corresponding to a velocity increase of one foot per second in 0.004 sccond. (See fig. 2-30.) Since the average velocity of the recoiling parts shortly after locking is approximately 18 feet per sccond, the time required to complete the remaining 0.060-foot movement to battery will be:

Therefore, the recoiling parts reach the battery position at 0.0490 second. Note that the velocity of the recoiling parts at the battery position is 18.2 feet per second.

With a velocity of 18.2 feet per second, the kinetic energy of the recoiling parts will be:

This is a relatively large amount of energy and could causc an extremely severe shock if an attempt were made to stop the recoiling parts merely by permitting the barrel to strike the brcech casing. One way to handle this energy would be to provide a heavy buffer which is designed to dissipate a large percentage of the striking energy of the recoiling parts. An idea of the character of this buffer can be obtained by considering what average force it would be required to produce if it is to stop the rccoiling parts within 0.250 inch (0.0208 foot). This force would be o^os2=12,350 P°unds

It must be emphasized that the buffer should be of a type which dissipates practically all of the energy it absorbs so that the recoiling parts will be braked to a stop and will not tend to rebound from the buffer. If the buffer docs not dissipate the striking energy, the residual energy may cause instability of action and may give rise to oscillations which can cause serious damage to the weapon.

As explained in the analysis of the short recoil system of operation, the problem of handling the kinetic energy possessed by the recoiling parts as they return to battery can be greatly simplified by timing the ignition of the next round so that the round is fired while the recoiling parts are still moving for ward. In this way, the large forces exerted in the early part of the propcllant explosion can be utilized to stop the rccoiling parts or at least can be used to aid the buffer.

Although the force of the propcllant explosion is capable of stopping and reversing the motion of the recoiling parts without the assistance of a buffer, there is an important point to consider. The final counter-recoil velocity of the gun used as an example is 18.2 feet per second and therefore the momentum of the 50-pound mass is:

Early in this analysis, it was pointed out that the total impulse which can be produced by the sample cartridge is only 35 pound seconds. Therefore, it appears that if the propcllant explosion were used alone to stop the forward motion of the recoiling parts after firing the first round, the impulse remaining to produce recoil would be only 35.0—28.3= 6.7 (pound seconds). Since the impulse producing recoil is now reduced so greatly, the next cycle of operation would be much slower than the first. However, if the gun continued to operate, it would soon settle down to some intermediate rate at which a dynamic equilibrium is established. The exact nature of the process is not important here, but the significant point to be noted is that the rate of fire of the gun will be decreased because of the fact that a considerable portion of the explosive impulse of the propcllant is used to stop the forward motion of the rccoiling parts. Thus the advantage gained by utilizing the force of the explosion to provide a buffer action can be obtained only by sacrificing speed of operation.

To avoid excessive loss of firing rate, it is generally advisable to stop the forward motion of the counter-recoiling parts by means of the explosive force of the next round. A buffer should also he used so that the momentum of counter-recoil will be partly cancelled before the next round is fired. With this combination of actions, the forward motion of the recoiling parts can be stopped smoothly without the necessity of having an impractically heavy buffer and without the excessive loss of the explosive impulse required for producing a high recoil velocity.

Proper functioning under these conditions will require high precision in timing the firing. Firing too soon will mean that the buffer will not contribute its

0 0