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Figure 3-24. Changes in Bolt Velocity Imparted by Blowback and Determination of Time for Unlocking.

(maximum at.00826 sec-225 ft/sec.)

total area-0206 ft.

increase in bolt velocity (18.2 ft/sec.)

are used is explained later in connection with plotting the theoretical time-travel and time-velocity curves for the interval before the piston strikes the bolt.

3. Selection of barrel spring characteristics and determination of counter-recoil velocity

Having determined the free recoil condition, the effect of blowback, and the unlocking time, the next step in the analysis is to select the characteristics of the barrel spring and to determine the counter-recoil velocity possessed by the gun at the instant it is fired just before reaching the battery position (advanced primer ignition).

In order to make use of advanced primer ignition it is necessary to sclcct the banrel spring characteristics carefully to insure proper timing of the counter-recoil motion of the gun. If the spring is too strong, the gun will reach the firing position too early and will be stopped by the mechanical buffer before the bolt can return, with the result that the advanced primer ignition effect will be lost. If the spring is too weak, the barrel will return too slowly, thus reducing the rate of fire. Within reasonable limits, however, the choicc of the spring characteristics is not absolutely critical because the timing can be modified slightly by adjusting the length of the recoil movement.

The required spring characteristics are determined as follows: A reasonable recoil travel for the gun is approximately 1.5 inchcs or 0.125 foot. Allowing for the fact that the gun is fired a short distance before it reaches battery, the counter-recoil travel before firing will be taken as 0.100 foot. At a rate of fire of 1200 rounds per minute, the time available for the gun to move through this distance in counter-recoil is somewhat more than half the cycle time or approximately 0.030 second. Therefore the average velocity of countcr-rccoil must be:

Under the action of the spring, the counter-recoil velocity will not change al exactly a constant rate

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