Kwd

.002 .004 OOB .008 .010 TIME (SEC.) Figure 17. Bolt Velocity Versus Time in a Plain Blowback Gun.

a few feet per second. Furthermore, from the shape of the curve in fig. 1-7, it can be seen that even the maximum bolt velocity attained will not be much greater than this value.

The second important point is that with such a low bolt velocity allowable, a high-powered plain blowback machine gun could never attain a reasonable rate of fire. To illustrate this point, the bolt of a 20-mm gun must open about. 10 inches in order to permit feeding. Thus, in opening and closing, it. must travel a total distance of nearly two feet per cycle, and if it. does this at ari average velocity of about two feet per second, the firing rate will be the ridiculously low figure of approximately 50 or 60 rounds per minute.

There are other difficulties encountered with a high-powered plain blowback gun and, in fact, these difficulties are so serious that it is difficult to make such a gun function as an automatic weapon. These difficulties are examined further in the following paragraphs. It might seem that such an analysis would amount to an unnecessary preoccupation with an impractical system. However, although this analysis deals with exaggerated conditions, it is made intentionally to disclose and highlight the fundamental concepts involved in the blowback principle and to provide a basis for understanding the other forms of blowback.

Fig. 1-8 shows the condition existing in a blow-back gun a few ten-thousandths of a second or so after the ignition of the propellant chargc. The pressure of the powder gases is driving the projcctilc forward and at the same time is driving the cartridge case and bolt to the rear. At this time, the chamber pressure is approximately 45,000 pounds per square inch and the forcc driving the projcctilc and cartridge ease will be in the neighborhood of 22,000 pounds (for a typical 20-mm round). Of course, some of the driving force is expended in overcoming the friction resisting the motion of the projectile and an additional portion of the driving forcc is used in imparting rotation to the projectile as it is spun in the barrel by the rifling. At. the breech end, similar losses occur in overcoming the fric-tional resistance to the motion of the cartridge ease and bolt and in compressing the driving spring. However, these losses, at the very most, can amount to only two or three, thousand pounds. This means that the remaining forcc of nearly 20,000 pounds is applied entirely in producing the forward acceleration of the projectile mass and the rearward acceleration of the bolt mass. In other words, the only-significant factor affecting the motion of these masses is the inertia of the projcctilc and of the bolt. Therefore, for purposes of simplified analysis, the frictional losses, the losses to projectile rotation, and the resist-

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