from maximum to zero in the manner illustrated means that its average velocity will only be slightly greater than one-half its maximum velocity. In other words, if this type of action were used in a short recoil gun, in spite of all the pains taken to achieve a high initial bolt velocity, the overall travel of the bolt would be accomplished at a much lower average velocity.

To overcome this disadvantage, the bolt driving spring can be made relatively very light so that it offers a low retardation and will permit the bolt to move its entire recoil distance with little loss in velocity. In this case, the function of the driving spring is merely to provide a positive force which is just sufficient to insure that the bolt will close. Stopping the bolt at the end of its travel and reversing its motion can then be accomplished by causing the bolt to rebound from a so-called "backplate buffer5'. This device is in effect an extremely stiff spring which absorbs all of the kinetic energy of the holt over a very short distance and then delivers energy back to the bolt to propel it forward. The reversing action produced by the backplate is so abrupt that the effect may be classified as an elastic impact.

In order to obtain a high rate of fire, it is also important for the bolt return to be accomplished at high velocity. If there were no energy losses involved in the reversing action, the forward velocity of the bolt after leaving the backplatc would be equal to the velocity at which it strikes the back-plate. This would be the ideal condition. However, in actual practice the coefficient of restitution for the bolt and backplate is usually considerably less than unity and the best that can be cxpcctcd is a coefficient in the neighborhood of 0.60 or 0.70; that is, the velocity after impact will be 60 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 affected 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 devicc which has the primary purpose of dissipating impact energy rather than of conserving

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