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Figure 2-22.

Changes in Bolt Velocity Imparted by Blowback and Determination of Time for Unlocking.

curve will show the momentum of these parts at any instant or, since the momentum is equal to the applied impulse, the curve will also show the impulse applied up to any instant. Now, after unlocking occurs, the impulse shown by this curve will be applied in changing the velocity of the bolt and therefore it is possible to divide each ordinate of the impulse curve by the bolt mass to obtain the new curve shown in fig. 2 22. (Only a portion of the vertical scale is shown in order to produce the significant portion of the curve in a large size.)

The actual velocitv values shown bv this curve are

meaningless but between any two values of time, the curve does show accurately what change in bolt velocity would be produced by the impulse.

Having the curve of fig. 2-22, it is only necessary to determine where to place the zero velocity axis so that the area between this axis and the curve up to 0.005 second is equal to 0.250 inch or 0.0208 foot. (Since this is a velocity-timc graph, areas under the curve represent displacement.) The zero axis can be located quite simply by drawing a line along the 0.005-second ordinate and measuring the area between this line and the curve, taking the elements of area as shown in the figure and working downward until the area is equal to 0.0208 foot. The abscissa of the point where the line bounding the lower limit of this area intersects the curve is the required time of unlocking (0.00307 second). Ordinates measured above the lower bounding line are equal to the free recoil velocity, with respect to the barrel, imparted to the bolt by blowback. The curve shows that the gain in free bolt velocity between the time of unlocking and 0.005 second is 18.2 feet per second.

It should be noted that although this computation neglects the retarding effect of the bolt driving spring, the resulting error is extremely small and entirely insignificant.

The data shown in fig. 2- 22 arc used in the computations for completing the bolt motion curves up to 0.005 second. The manner in which the data are

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