  Pure theory recognizes the dependence of supersonic drag coefficients on both
Mach number and Reynolds number, which includes an implicit dependence of drag
coefficient on air density. However, many modern approaches to computing
trajectories for artillery and small arms treat drag coefficients as a function
of Mach number and assume no dependence on Reynolds number. If drag force is
strictly proportional to air density for supersonic projectiles (as suggested
by applied theory), the drag coefficient should be independent of air density
over a range of Mach numbers. Experimental data to directly support this are
not widely available for supersonic projectiles. The experiment determined drag
on a 2.59 g projectile from M1.2 to M2.9 using optical chronographs to measure
initial and final velocities over a separation of 91.44 m. The free flight
determination of drag coefficients was performed at two significantly different
atmospheric densities (0.93 kg/m3 and 1.15 kg/m3 ). This experiment supported
direct proportionality of aerodynamic drag to air density from M1.2 to M2.9
within the experimental error of 1%-2%.
