same as that of a very heavy bolt, a retarded blow-back gun would suffer from all of the disadvantages of a plain blowback gun (except for the disadvantage of excess weight). These disadvantages, which are explained in detail under "Plain blow-back system,': are principally a very low rate of fire and insufficient bolt energy for operating the gun mcchanism. Therefore, it appears that there is little point in using the retarded blowback system merely as a means of saving weight.

The real advantage of using retarded blowback can be obtained only if the retardation characteristics of the mechanism are not uniform. The ideal type of retarded blowback mechanism would be one in which a very high resistance to bolt movement is encountered during the period of high chamber pressure which exists until after the projectile has left the muzzle. This high resistance would result in a vcrv low bolt vclocitv and a verv small bolt

movement during the period of high pressure. It would then be desirable for the last part of the small bolt movement to cause a change in the characteristics of the retarding mcchanism so that the resistance is decreased bv a substantial amount a milli-

second or so after the projectile has left the muzzle and the residual pressure has dropped to a safe operating limit. The bolt, being subjected to greatly decreased resistance, would then be blown back safely at a relatively high velocity. An examination of the preceding requirements will in dicate that an ideal retarding mechanism approximates the effect of a delay mechanism except that the bolt is not rigidly locked at any time. (Gf. "Delayed blowback system".)

It is important to note that effective use of retarded blowback requires a high degree of precision of mechanism and timing of operation. For safety and proper functioning, the changc in the resistance offered by the retarding device must occur at exactly the correct time during the propellant explosion. This fact makes barrel length the most critical factor to be taken into consideration. For example, it has been stated that in the Schwarzlose machine gun (which employs a toggle linkage for retardation) the barrel length is critical to an unbelievable degree. If the barrel were slightly longer, the residual pressure would be too high as the linkage rises. Since the retardation would then be too low for the existing pressure, this would create a condition of extremely violent recoil and possible explosive rupture of the cartridge case. If the barrel were slightly shorter, the residual pressure would drop excessively before the linkage has risen sufficiently. Since the retardation would then be too high for the available pressure, there would be insufficient recoil and the gun would fail to function.

The foregoing example serves to indicate that a retarded blowback gun for high-powered ammunition must be designed and developed with great care. The parts of the retarding mechanism must be well

Figure 1-45. Relationship of Velocities in Toggle Linkage.

Figure 1-45. Relationship of Velocities in Toggle Linkage.

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