## Recoil Operation

The basic controlling factor is the limit on the allowable movement (0.250 inch). If the time in which this movement is accomplished is long, the average velocity of the movement will necessarily be low. However, if the time for the movement is made very short, the average velocity may be very large. For example, suppose the bolt is unlocked 0.002 sccond before the safe pressure of 750 pounds per square inch is reached at 0.005 second. The bolt can now travel the 0.250 inch in 0.002 second. That is to say, its average velocity for this interval can be:

Now assume that the bolt is unlocked only 0.001 second before the safe pressure is rcachcd. It can then travel the 0.250 inch at an average velocity for the interval of:

The foregoing indicates that, by shortening the time for which the blowback operates before the safe pressure is reached, increased bolt velocity can be achieved without exceeding the allowable bolt movement.

Of course, it should be realized that shortening the time of blowback action reduces the blowback impulse available for producing the velocity. Thus, in order to gain an increase in allowable average velocity by reducing the time of action, it is necessary to reduce the bolt weight. To illustrate this point, if the velocity of 10.4 feet per second cited in the first example above was obtained with an 8-pound bolt, it would be neccssary (with the same cartridge and gun) to rcduce the bolt weight by a factor of at least 4, even if it is assumed that the average blowback pressure is the same for both examples. Actually, since the residual pressure decreases with time, the average pressure for the second example would be considerably less than for the first and therefore a further reduction in bolt weight would be required. The actual weight reduction factor would be more nearly in the neighborhood of 6, giving a bolt weight of only 1.3 pounds. (This would probably be much too light for a practical gun.)

Thus, it appears that, a substantial gain in average allowable bolt velocity by means of reducing the time of blowback action can be achieved only by a drastic reduction in boll weight. Although it is not practical to attempt to reduce bolt weight by an excessive amount, it is important to note that efficient utilization of blowback in a short recoil gun can be of great advantage in attaining the high bolt velocity necessary for a high rate of fire. However, this advantage can be gained only through precise timing of unlocking in combination with careful attention to minimizing bolt weight.

The third subject for consideration is the action of the accelerating device. From what has been said in the preceding paragraphs about the effects of blowback, it can be seen that the action of the accelerator should be dclaved until after the residual

pressure has dropped to the safe limit there specified (750 pounds per square inch for the sample conditions). If this delay is not provided, the effect of the accelerator will be wasted up until the time the pressure has dropped to the safe limit. As has been explained, the velocity of the bolt must be limited so that the bolt movement does not exceed 0.250 inch while the pressure is above 750 pounds per square inch. Since blowback alone can easily impart the velocity required to produce the 0.250-inch movement, assistance from the accelerator is not necessary or desirable. However, once the pressure has fallen below 750 pounds per square inch, rupture of the cartridge case will not occur and the need for limiting the bolt velocity no longer exists. At this point, then, the accelerator can start its action and further increase the velocity of the bolt.

There arc several important points concerning the action of the accelerating device which should be considered at this time. First, the device should be designed carefully to act smoothly and positively so that it can transfer velocity to the bolt without excessive shock or friction. This is particularly necessarv because even at best the action of the

accelerator usually must be completed in a few-thousandths of a second and will therefore be quite violent. If the action is not made as smooth as possible, battering of the parts, deformation of the mechanism, and frequent breakages will he unavoid

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