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12 X.0016(5

\sec

As shown in fig. 1-31, the residual pressure continues to act for some time after 0.00500 second and therefore the final velocity reached by the bolt can be considers ly higher than the 12.5 (ft./scc.) average value allowable for the first 0.00166 second after the bolt is unlocked. The foregoing considerations show that the use of delayed blowback permits the attainment of relatively high bolt velocities with conventional ammunition so that it is not necessary to use the special ammunition described for advanced primer ignition.

When delayed blowback is used, only a very small portion of the total impulse of the propellant explosion is available for operating the gun mechanism. The relative magnitude of the available portion of the total impulse can be visualized by-examining fig. 1-32, which shows the variation of the chamber pressure with time. Assuming that the bolt is unlocked 0.001 second after the projectile leaves the muzzle, the shaded area in this figure (when multiplied by the bore cross-section area) would represent the impulse which produces the relative velocity between the bolt and the barrel.

The unshaded area under the curve (also multiplied by the bore cross-section area) would represent the impulse applied to the barrel and bolt while these parts are locked together.

The points made in the preceding paragraph indicate that the bolt must be relatively light in order for the small available impulse to producc a high bolt velocity and sufficient bolt energy. Also, since such a great impulse is applied before the barrel and bolt are unlocked, some means must be provided to account for the recoil reactions which occur before unlocking. Of course, in a back gun employing recoil unlocking, the impulse before unlocking is utilizxd to producc the recoil motion of the barrel and bolt. In guns employing gas unlocking, primer actuation, booster actuation, or some other form of unlocking, it is

necessary to mount the barrel so that it can recoil to the rear and thus provide a means for controlling the magnitude of the trunnion reaction.

It should be pointed out here that the barrel and bolt, while locked together, can acquire a considerable recoil velocity with respect to the breech casing of the gun. After unlocking occurs, the blowbac k action drives the bolt to the rear and imparts an additional velocity to the bolt. Therefore, the actual velocity of the bolt measured with respect to the breech casing is the sum of the velocity of the barrel at the instant of unlocking and the velocity imparted by blowback. For this reason the bolt

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