All of the major difficulties associated with the design of blowback machine guns stem from excessive movement of the cartridge case during the time of action of the powder gas pressure. The preceding paragraphs show how the use of advanced primer ignition causes the initial high-pressure phase of the propeliant explosion to be expended in stopping the forward motion of the bolt, with the resulting advantages of a lighter bolt and a slightly higher rate of fire than would be permissible with plain blowback. These advantages are attributable to the fact that the rearward motion of the bolt is delayed for approximately one-thousandth of a second. thus halving the impulse which is effective in blowing back the bolt and also reducing the time during which the bolt is accelerated to the rear. Since this time is reduced, the average rearward velocity of the bolt can be higher during the interval without exceeding the allowable movement.
The beneficial effects of the delayed rearward
bolt movement obtainable with advanced primer ignition lead to the conclusion that still greater benefits could be achieved by further delaying the movement. Unfortunately, the delay which can be
obtained with advanced primer ignition is more or less fixed and amounts to the time required for the propeliant explosion to produce approximately one-half of its total impulse. (In other words, for a 20-mm gun such as used in the examples previously discussed, the delay is approximately 0.0009
to 0.0010 second.) To delav the rearward motion
' j of the bolt beyond this time, it is necessary to resort to a special operating system referred to as "delayed blowback".
Delayed blowback mav be defined as the system
of operation in which the bolt remains locked until the peak powder gas pressures have passed and a safe operating limit is reached after the projectile clears the muzzle. The bolt is then unlocked by-some means so that it can be blown back bv the residual pressure with sufficient energy to perform the remainder of the cycle of operation. In this system, the time at which the bolt is unlocked can be controlled in the design so that any desired portion of the residual pressure can be utilized. Of coursc, the bolt must be unlocked while there is still sufficient impulse available from the residual pressure to produce the required operating energy.
Any gun in which the bolt is unlocked while there is still some residual pressure is subject to some blowback and partakes of some of the characteristics of the delayed blowback system. Such guns includc certain gas-operated and short-recoil-oper-ated weapons in which the bolt is unlocked almost immediately after the projectile has left the muzzle. However, in guns of this type, the operating energy does not conic primarily from blowback hut is derived mainly from the action of the gas piston or from the motion of the recoiling parts; therefore these guns will not be described at this time. The present analysis is conccrncd only with guns in which delayed blowback is the main source of energy.
The methods which have been used to unlock the bolt in delayed blowback guns arc very numerous. Some are relatively simple and some quite elaborate but all arc intended to keep the breech rigidly locked for a portion of the time of action of the powder gases. In some guns, the barrel and loeked bolt are permitted to recoil together for a short distance and this motion is then utilized to perform the operation of unlocking. In other guns, the barrel is tapped so that a portion of the expanding powder gases can be by-passed to operate a piston or lever which actuates a mechanism so arranged that the breech is unlocked shortly after the projectile leaves the muzzle. Another method, known as primer actuation, uses an arrangement whereby the chamber pressure causes the primer of the cartridge to move to the rear in its pockct. As the primer is set back, it impinges on a sliding member and drives
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