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

during the reversal of the bolt motion at the end of the rccoil stroke.

Since the ob ject of utilizing blowback is to impart velocity to the bolt, it is of interest to determine what conditions lead to the attainment of high blowback velocities. The problem involved in determining these conditions is how to obtain the high velocity and yet not exceed the allowable bolt movement (0.250 inch for the assumed conditions). 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 second before the safe pressure is reached. The bolt can now travel the 0.250 inch in 0.002 second. That is to say, its average velocity for this interval must be limited to:

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