Allowable Movement

Figure 2-16. Limit of Cartridge Case Movement Rearward Before Residual Pressure Reaches Safe Value.

mcnt will be exceeded before the pressure drops to the safe limit of 750 pounds per square inch. If unlocking is delayed too long, the impulse imparted to the bolt will be unnecessarily small and the full benefit of blowback will not be realized. The ideal unlocking time for a given bolt weight is that which will permit the bolt to move the full allowable 0.250 inch and no more by the time that the pressure has dropped to 750 pounds per square inch.

It is appropriate here to consider the effect of bolt weight a.s it. is related to the blowback action in a short recoil gun. First, the primary purpose in making use of blowback is to obtain an increase in bolt velocity and therefore it is obviously desirable to set up the design so as to make the best possible use of the available blowback action. As has been explained in the preceding paragraphs, it is possible for blowback to produce extremely high bolt velocities but unfortunately it is necessary to limit the bolt motion in order to avoid rupture at the base of the cartridge case. (The limit assumed for the sample conditions is a bolt motion of 0.250 inch up to the time that the residual pressure has dropped to 750 pounds per square inch.) The question now at hand is how to limit the bolt movement as required and yet obtain a high velocity. The answer to this question lies in the selection of bolt weight and unlocking time.

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