A line representing this value of free bolt momentum is drawn on the bolt momentum graph (fig. 1-13) and the curve previously drawn is extrapolated until it becomes tangent to the line. The point at which the curve is tangent represents the time at which the residual pressure becomes zero and therefore imparts no further momentum to the bolt. 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 plotting to determine this point by using Vallier's formula for approximating the duration of the residual pressure:

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