The Machine Gun

11. Return motion oj barrel and bolt during action of accelerating lever

During the return motion of the barrel and bolt, the effect of the action of the accelerating lever is to transfer kinetic energy from the bolt to the barrel, thus gradually slowing the motion of the bolt and accelerating the motion of the barrel. This action has the beneficial effect of greatly reducing the shock which would occur if the bolt, which is returning at high velocity, merely collided with the rear face of the barrel.

The analysis of the motion of the barrel and bolt during the action of the accelerating lever in counter-recoil is considerably more complicated than for recoil. This is because of the fact that the lever has already been designed and the velocity of the barrel or bolt can no longer be selected as desired but will be determined by the characteristics of the lever.

The analysis during the action of the accelerating lever is conducted on the basis of equating the energies in the system. The energy relationship may be expressed as:

Initial KE+Work done by springs—Energy losses=Barrel KE+Bolt KE.

This relationship, considered together with the fact that the lever causes a known relative motion between the barrel and bolt and also causes a known ratio between their velocities, is used as follows in determining the motions:

At the start of the action of the lever, the velocities of the barrel and bolt are known. That is (from fig. 2-30) feet per second and V2=48 feet per second. Note that the ratio between these velocities is 10:1 which is the same as the lever ratio of the accelerator at the instant it is engaged. (Cf. fig. 2-28.) This is a desirable condition because it means that the action of the lever will start without a large shock. With the stated velocities, the initial energies in the barrel and bolt will be:

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