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Figure 3-14. Variation of Bolt Velocity With Time When Driving Spring Absorbs All of Bolt Energy.

of the bolt after leaving the backplate would be equal to the velocity at which it strikes the backplate. This would be the ideal condition. However, in actual practice the coefficient of restitution for the bolt and backplatc is usually considerably less than unity and the best that can be expected is a coefficient in the neighborhood of 0.60 or 0.70; that is, the velocity after impact will be GO or 70 per cent of the velocity before impact. This represents satisfactory performance, but if the coefficient of restitution is too low as the result of poor backplate design, the return of the bolt will be sluggish and the rate of fire will be afïccted adversely. In this

connection, it should be emphasized that the purpose of the backplate buffer is to reverse the motion of the bolt with as little loss of energy as possible. In many instances, the term "buffer" is used to refer to a device which has the primary purpose of dissipating impact energy rather than of conserving energy to produce an efficient rebound action. For this reason it might be better to refer to the back plate buffer as a "bolt dcflcctor" or "rebound plate".

The conditions under which the bolt rebounds from the backplate in a gas-operated gun are extremely important and must be considered carcfully in the design. In dealing with these conditions, it is essential to make proper allowances for the fact that the entire gun moves in recoil and to time correctly the motions of the various parts. The necessity for careful timing arises principally from the fact that the backplate buffer is mounted on the gun and accordingly moves with the gun in recoil. Since the position of the backplate is not fixed, it is possible to have several different conditions of relative motion and impact, depending on the particular timing used in the design.

The timing involved in this instance is primarily dependent on the motion of the gun in its cradle and is therefore mainly controlled by the action of the barrel return spring and the recoil buffer. If the spring and buffer stop the recoil motion of the gun and start to return the gun to battery before the

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