Gas Operation

First, the mechanism should be arranged so that the bolt will be unlocked as soon as the chamber pressure has reached a safe operating limit. This will aid in obtaining a high rate of fire by eliminating any unnecessary delay and will also make it possible to derive useful bolt energy by utilizing the blowback effect available from the residual powder gas pressure. In order to facilitate extraction and to make the best use of blowback, it is important to avoid binding or excessive friction between the cartridge case and chamber wall after unlocking occurs. With lubricated ammunition, friction and binding do not present a problem, but with unlubricated ammunition, the cartridge ease will always tend to seize in the chamber. This binding occurs because the peak chamber pressures and the heat of the explosion expand the ease tightly against the chamber, and since unlocking takes place while there is still an appreciable residual pressure, the case does not have a chance to contract sufficiently to permit it to move freely under blowback. This difficulty can be avoided by employing an operational feature known as "initial extraction". When this feature is incorporated in the unlocking mechanism, the bolt is unlocked in two stages. In the first stage, the bolt is not unlocked completely but is cammed powerfully back through just a sufficient distance to cause the taper of the cartridge case to break free of the chamber wall. Immediately thereafter, the bolt is unlocked completely and blowback can occur without difficulty.

The utilization of blowback is limited by the fact that the cartridge case can not be permitted to move too far out of the chamber while the residual powder gas pressure is still high enough to cause rupture of the ease near the base. The actual limit on the amount by which the cartridge case can be permitted to move as it is related to the chamber pressure will of course depend on the specific cartridge case under consideration. A good way to estimate the limit for a given cartridge case is to consider what pressure oould be withstood by the case when the case has moved just far enough to the rear to expose the thin walls near the base. (See fig. 3-9.) For an ordinary 20-mm cartridge case, this occurs when the case has moved approximately 0.250 inch to the rear. When the case has reached this position, it is reasonable to assume that the internal pressure should not be in excess of 750 pounds per square inch, in order to be sure that the case barrel bolt

allowable movement

Figure 3-9. Limit of Cartridge Case Movement Rearward Before Residual Pressure Reaches Safe Distance.

will not be ruptured. Fig. 3-10, which is a graph showing the residual pressure variation with time for the assumed gun and cartridge, indicates that the pressure does not fall to 750 pounds per square inch until 0.005 second after ignition of the pro-pellant charge. (Of course, in any practical application, the actual limiting values for a cartridge ease should be determined by experiment.)

Having a bolt with a given weight, the bolt movement can be limited as required only by selecting the proper instant for unlocking. If the bolt is unlocked too soon, it will receive too great an impulse from blowback and its average velocity will be so great that the allowable movement will be exceeded before the pressure drops to the safe limit. 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 distance and no more by the time that the pressure has dropped to the desired level. After the pressure becomes less than this value, there is no further danger of case rupture and the movement of the case need not be limited. In fact, from this point on, there is no reason for limiting the bolt velocity except to avoid excessive violence of action and exorbitant breakage of parts which would occur

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