Data from rapid penetration tests into granular media were analyzed using a two-step Poncelet equation to determine values for the Poncelet drag coefficient on a projectile. The data were from a spherical projectile launched into both densely and loosely packed Ottawa sand at transonic impact velocities of approximately 300 m/s. Velocity data were obtained and integrated over time to produce data for penetration depth. Three approaches were investigated: (1) Fitting the data with a constant drag coefficient, (2) fitting the data with a constant drag coefficient along with a constant velocity-independent term computed as the average of the quasi-static penetration resistance into the same target, and (3) by separating the data into two segments about a transition velocity, and using best fit values for the inertial drag and bearing stress terms. The latter approach produced superior fits compared to the other two procedures investigated. The result indicates a velocity dependence of both the inertial drag coefficient and bearing stress on a projectile during rapid penetration into granular media.