Recoil Operation

Shortly after unlocking occurs, the barrel (which is still moving to the rear of its own momentum) strikes the accelerating device (fig. 2-14C). This device can take many forms, but in the mechanism illustrated, it is a lever pivoted in the breech casing. Fig. 2-15 shows the action of this lever in detail. In fig. 2-15A, the barrel extension is just in engagement with the lever and in fig. 2-15ft the lever has been rotated to where it has started to thrust on the bolt. As the barrel continues moving to the rear, the lever rotates and the point of contact between the barrel extension and the lever moves closer to the lever pivot (fig. 2-15C). This causes the top of the lever to move more rapidly, thus imparting a high acceleration to the bolt.

When the accelerating action is completed, the barrel strikes a buffer stop which absorbs the remaining recoil energy in the barrel and its associated parts. After the barrel has been stopped, it is latched in its rearward position so that it is not immediately driven forward to battery by the compressed barrel return spring.

The combined action of the accelerating device and the blowback produced by the residual pressure imparts a high velocity to the bolt and then the bolt continues to move to the rear of its own momentum 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 compjressed. This spring is relatively light and its only function is to assist the return motion 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. The forward velocity of the bolt immediately after leaving the backplate is somewhat less than the velocity at which it strikes the backplate because the impact is not purely elastic and some energy is lost as heat in the exchange.

As the bolt moves forward, 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. Just before the bolt locks to the barrel, the barrel is unlatched so that the bolt ?rid barrel move forward into battery while locked together. Shortly before the recoiling parts reach their most 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 arc then driven to the rear. (Timing the firing in this way eliminates the need for a heavv counter-recoil buffer to absorb and dissipate the forward kinetic energy of the recoiling parts.)

Analysis of Short Recoil

The most outstanding feature of the short recoil system of operation is that by proper design, very high cyclic rates can be attained. The bolt is unlocked without unnecessary delay shortly after the projectile has left the muzzle and then the bolt, which is already moving with considerable recoil velocity, is propelled to the rear at even greater velocity by the combined effects of the accelerator and blowback. With this high bolt velocity, the recoil movement of the bolt and its return to battery

arc accomplished in a very short time.

From the foregoing, it is evident that the bolt velocity in a short recoil gun is determined by three separate factors: (1' the recoil velocity at the time of unlocking, (2) the additional velocity imparted by blowback, and (3) the additional velocity resulting from the action of the accelerating device. In the following paragraphs, each of these factors is analyzed separately.

The recoil vclocitv of the bolt at the instant of t unlocking is the result of the total impulse applied by the powder gases to the combined mass of the barrel and bolt, while these parts are loeked together. For any particular cartridge, the total forward momentum of the projectile and powder gases will producc an equal and opposite momentum in the recoiling parts. The recoil velocity resulting from this momentum will be inversely proportional to the weight of the recoiling parts; that is, the lighter the recoiling parts are, the higher will be their velocity. Therefore, within reasonable limits, to obtain a high recoil velocity while the barrel and bolt are still

0 0

Post a comment