note- movement between barrel and bolt =.0208 ft. at .005 sec.
Figure 2-24. Development of Time-Travel and Time-Velocity Curves for Period Between Unlocking and
Start of Acceleration.
therefore there will be some small additional blow-
back effect which will occur simultaneously with the
action of the accelerator.
The first point to consider is the amount of kinetic energy available from the moving mass of the barrel. Sincc the barrel weighs 45 pounds and at 0.005 second is moving at a velocity of 18.5 feet per second, the kinetic energy possessed by the barrel mass is:
These 239 foot pounds of barrel energy do not constitute all of the energy which can be utilized to accelerate the bolt bccause some work will be done on the bolt as the result of the force exerted by the residual pressure. Although this force can be determined at any instant from the graph of residual pressure versus time (fig. 2-17), it is not possible at this time to determine the work done by this force because the bolt displacement for the interval of acceleration is not yet known. However, the distance moved by the bolt during the 0.003 second for which the residual pressure will continue to act will certainly be quite short and the average force exerted by the powder gas pressure during this movement will also be relatively small. These conditions indicate that the work done on the bolt by the last portion of the residual pressure will be of little consequence when compared with the 239 foot pounds of energy available from the barrel mass. A rough estimate indicates that the work which can be done on the bolt bv the remain-
ing blowback action will be in the order of only 10 or 15 foot pounds. Sincc this amount of work is so small, little error is likely to occur if it is assumed for the present that the energy available for acceleration is 250 foot pounds.
It is of interest to consider what the result would be if the efficiency of energy transfer during acceleration were 100 per cent (that is, if all of the available energy could be transferred to the bolt
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