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0 .001 .002 003 .004 005 .006 .007 .008 .009 time (sec)

Figure 3 23. Impulse of Chamber Pressure.

pulse imparted up to any instant by the action of the chamber pressure. (That this is true may be seen by recalling the basic relation, I-MV.) The reason for using the special curve of fig. 3-19 to determine the impulse is that only the chamber pressure acts on the bolt. Since the piston impulse is applied only to the gun, it should not enter into the calculations for the blowback action.

Now, after unlocking occurs, the impulse shown by fig. 3-23 will be applied almost entirely in changing the velocity of the bolt. Therefore it is possible to divide cach ordinate of the impulse curve by the bolt mass to obtain the new curve shown in fig. 3 24. (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 velocity values shown by this curve are meaningless but between any two values of time the curve does show what change in bolt velocity would be produced by the impulse.

Having the curve of fig. 3-24, 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-time graph, areas under the curvc 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 the line and the curve, taking the elements of area as shown in the figure and working downward until the area is 0.0208 foot. The abscissa of the point where the line bounding the lower limit of this curve intersects the curve is the required time of unlocking (0.00307 second). Ordinates measured above this line arc 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 sccond.

It should be noted that although this computation neglects the effect of the bolt driving spring on the 0.250-inch travel, the resulting error is extremely small and entirely insignificant.

The data shown in fig. 3-24 are used later in the computations for completing the bolt motion curves up to 0.005 sccond. The manner in which the data

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