## Blowback

Figure 1-32. Variation of Chamber Pressure With Time Showing Proportion Available lor Blowback

(Breech Unlocked 0.001 Second After Projectile Leaves Muzzle).

velocities obtained with delayed blowback are {airly great and accordingly guns employing this principle are capable of relatively high rates of fire.

### Mathematical Analysis of Delayed Blowback

The following mathematical analysis of delayed blowback is based on the same general principles used for the analysis of plain blowback except that the problem must be considered in two stages. These stages cover the conditions which exist before unlocking and the conditions which exist during and after blowback occurs. In this analysis it will be assumed that the same ballistic data used for plain blowback are available. (See figs. 1-9, 1-10, and 1-11.) Since many of the methods and formulas employed in this analysis are the same as or very similar to those used for plain blowback, the derivations of the formulas and the explanations of the procedures will not be repeated here. However, as they arise, any new concepts or new formulas will be explained.

In order to perform an analysis of delayed blow-back, it is necessary to assume certain definite characteristics of the gun to be used as an example. First, it will be assumed that recoil unlocking will be employed. Since both the barrel and bolt move in recoil before unlocking, the weight of these parts must be known in order that their motion characteristics may be determined. In an actual design problem, it would first be necessary to design the barrel and plan the mechanism sufficiently at least to permit making a preliminary estimate of the weight of the recoiling parts. For purposes of the present analvsis it will be assumed that the barrel and its

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