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mentum until the opening between the barrel and bolt is sufficient to permit feeding. As the bolt moves back, the spent cartridge case is extracted from the chamber and ejected and the bolt driving spring is compressed. This spring is relatively light and its only function is to assist the return movement of the bolt. Therefore, the driving spring does not absorb any great portion of the kinetic energy of the recoiling bolt and the bolt moves through its entire recoil distance at high velocity. The bolt then strikes the backplate buffer and rebounds (fig. 3-6D). The forward velocity of the bolt immediately after leaving the backplate buffer is somewhat lower than the velocity at which it strikes the backplatc bccause the impact is not purely elastic and some energy is lost as heat in the exchange.

NOTE: It is important to realize that the velocity with which the bolt strikes the backplate buffer will depend on the condition of motion of the gun at the instant of impact. If the gun happens to be moving forward at this instant, the impact velocity will be higher than if the gun were stationary or moving rearward. It can be seen that if the gun motion is not controlled to insure uniform velocity at the instant of contact, the bolt impact will he entirely unpredictable and will vary widely from shot to shot. This could give very erratic performance and may result in a high incidence of parts breakage.

As the bolt moves forward after rebounding from the backplatc buffer, its motion is aided by the driving spring. The bolt picks up a fresh cartridge from the feed mechanism and loads this cartridge into the chamber. The bolt then locks into the barrel. Usually it is desirable to time the operation of the weapon so that the bolt returns and locks to the barrel before the gun has reached the battery position. Then, shortly before the recoiling parts reach their mast forward position, the firing mechanism is actuated and a new cycle begins. Since the cartridge is fired before the counter-recoil motion is completed, the forward velocity of the recoiling parts is first checked by the initial part of the rearward thrust exerted by the exploding propellant charge and the recoiling parts are then driven to the rear. (Timing the firing in this way eliminates the need for a heavy counter-recoil buffer to absorb and dissipate the forward kinetic energy of the recoiling part«;.)

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