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

The preceding examples indicate that, by shortening the time for which blowback operates before the safe pressure is reached, increased bolt velocity can be achieved without exceeding the allowance bolt movement.

Of course, it should be realized that shortening the time of blowback action reduces the blowback impulse available for producing the velocity. T herefore, 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 make up for the reduction in the impulse. To illustrate this point, if the velocity of 10.4 feet per second cited in the first of the preceding examples was obtained with an 8-pound bolt, it would be necessary (with the same cartridge and gun) to reduce the bolt weight by a factor of at least 4 to obtain the velocity of the second example, 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,

/ w giving a bolt weight of only 1.3 pounds. (This would probably be much too light for a practical 20-mm gun.)

Thus it appears that a substantial gain in average bolt velocity can be achieved by reducing the time of the blowback action, but this can be done only if

Figure 3-10. Graph of Residual Pressure Versus Time for a Typical 20 mm Gun.

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