In designing the spring so that it will producc this average force, the same factors described in the analysis of plain blowback should be considered. However, an arbitrary choice of spring characteris tics will suffice for present purposes. If the initial compression is taken as 120 pounds, a maximum force of 312 pounds will producc the required average force of 216 pounds. Since the difference between the maximum force and initial compression is 192 pounds and the recoil distance is 10 inches, the spring constant will be 19.2 pounds per inch or 230 pounds per foot. Realizing that this choice is arbitrary, it will be assumed here that the selected values of Fo= 120 pounds and K=230 pounds per foot will result in a satisfactory spring.

Having the characteristics of the bolt driving spring and the free recoil velocity curve of fig. 1-37, the time-travel and time-vclocitv curves for the bolt can be constructed using the same general approach used for plain blowback. At the instant unlocking occurs, the bolt driving spring has been compressed 0.0470 foot (fig. 1-34). This means that for considering the period of blowback, the initial compression of 120 pounds must be increased by the cffcct of the spring constant for this deflection. That is:

Fig. 1-38 shows the time-travel and time-velocity curves obtained for the bolt of the gun used as e\-

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