One of the principal disadvantages of the plain blowback system is the excessive bolt weight required to limit the movement of the cartridge case to a safe value during the action of the powder gas pressure. This particular difficulty can be circumvented by employing the "retarded" blowback system—a system in which a special type of retarding mechanism is operated by the movement of the bolt. The mechanism itself Is composed of relatively light parts and the inertia forces which result when these parts arc set in motion by the bolt are therefore relatively small. However, the mechanism is arranged so that the bolt must act through a tremendous mechanical disadvantage to overcome the inertia forces and is accordingly subjected to a very high resistance to motion. In other words, although the bolt and the associated mechanism may be quite light, the effective resistance to bolt acceleration can be made just as great as that which would be obtained by the use of a very heavy bolt.
The mechanism of a retarded blowback gun is similar to that of a plain blowback gun except for the presence of the retarding mechanism. This mechanism can take many forms but the basic principle underlying all of these forms is that the bolt must overcome the inertia forces in the mechanism by acting through a high mechanical disadvantage. One such mechanism is illustrated schematically in fig. 1-42. This mechanism is essentially a toggle joint and when the bolt is closed, the angle "a" indicated in the figure is nearly 180 degrees. Under these conditions, the linkage acts like a crank mechanism which is slightly off dead center and mast of the blowback force acting on the linkage is not effective in opening the bolt but is transmitted directly through the links to the breech
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