BARREL JACKET (FIXED)
(1) the recoil velocity imparted to the barrel and bolt while these parts are still locked together, (2) the additional velocity imparted to the bolt by blow-back after unlocking, and (3) the gain in bolt velocity produced by the accelerator. Since the principal reasons for using the short recoil system of operation is to obtain a high rate of fire, each of these factors is considered from the standpoint of how to achieve the highest possible bolt velocity consistent with safety and reliable functioning. The methods used to obtain a high bolt velocity may be summarized as follows: The total weight of the recoiling parts should be kept to a minimum so that the recoil velocity before unlocking will be high. Next, the bolt should be as light as possible and should be unlocked at the proper instant so that the blowback effect can produce a large increase in velocity without causing rupture of the cartridge case. Finally, the accelerating device should start to function just at the instant the residual pressure has reached a safe limit and should be designed to produce the maximum possible transfer of energy from the barrel to the bolt.
Another means of increasing the rate of fire in a short recoil gun is to employ a device known as a "muzzle booster' or "recoil intensifier". An ex ample of a dcvice of this type is shown in fig. 2-19 and other forms are illustrated in Part XI of this publication. The booster is installed at the muzzle of the gun and operates by trapping the muzzle blast in such a way as to apply a heavy thrust on the front faec of the barrel. This additional thrust causes the recoiling parts to have a higher velocity and hence increases the velocity inherited by the bolt as well as increasing the energy available for acceleration of the bolt. Also in some designs, it is possible for the trapped gas to produce a slightly greater blowback effect than would be obtained without the booster. Although the muzzle booster can be used with good cffcct to increase the rate of fire of a short recoil gun, it is important not to employ excessive boosting action. If the booster acts too powerfully, extremely violent recoil will result with the consequent danger of severe shocks and pounding.
All of the foregoing analysis is related to the methods which can be used to impart a high initial velocity to the bolt. It is now appropriate to consider the motion of the bolt after this velocity is imparted. The bolt is given its initial velocity early in its rearward travel and then it completes its motion of its own momentum. To permit feeding, the bolt must move to the rear through a distance at least as great as the overall length of the complete cartridge and then its motion must be reversed to load the gun and close the breech. In some guns, this action is accomplished through the use of a relatively powerful driving spring which is compressed as the bolt moves in recoil. The spring absorbs the kinetic energy of the bolt over the full recoil travel, finally stopping the rearward motion of the bolt when all of the kinctic energy of the bolt has been absorbed. The spring is designed so that this occurs when the opening is sufficient to permit feeding. The compressed spring then drives the bolt forward to complete the operating cycle. This type of design has a serious drawback from the standpoint of speed of operation. Since the bolt is gradually slowed down by the spring, its velocity varies from maximum at the beginning of recoil to zero at the end of recoil. (See fig. 2-1 1, which is a graph showing how the bolt velocity varies with time under these conditions/) The fact that the bolt velocity varies
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