  This paper describes the effects on aerodynamic drag of rifle bullets as the
gyroscopic stability is lowered from 1.3 to 1.0. It is well known that a bullet
can tumble for stability less than 1.0. The Sierra Loading Manuals (4th and 5th
Editions) have previously reported that ballistic coefficient decreases
significantly as gyroscopic stability, Sg, is lowered below 1.3. These
observations are further confirmed by experiments reported here. Measured
ballistic coefficients were compared with gyroscopic stabilities computed using
the Miller Twist Rule for nearly solid metal bullets with uniform density and
computed using the Courtney-Miller formula for plastic-tipped bullets. The
relationship between Sg and drag may be used to test the applicability of
existing gyroscopic stability formulas for given bullet designs and to evaluate
the accuracy of alternate formulas in cases where the existing stability
formulas are not as accurate. The most definitive test of formulas predicting
stability will always be observation of whether bullets tumble under given
conditions. However, observations of drag changes provide valuable supplemental
information because they suggest changes in stability as conditions change. Use
of a continuous variable (drag) rather than a binary variable (tumbling) allows
insight into stability over a range of conditions where the binary variable
does not change.
