A line representing this value of the maximum velocity of free recoil is drawn on the velocity graph (fig. 3-21) and the curve previously drawn from equation 3-16 and fig. 3-8 is extrapolated until it becomes tangent to the line. The point at which the curve becomes tangent represents the time at which the residual pressure becomes zero and therefore imparts no further velocity to the recoiling parts. Although an error in locating the exact point of tangency will not have any serious effect on the accuracy of the results, it may be of some assistance in drawing the curve to determine this point by using Vallier's formula for approximating the duration of the residual pressure:

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