The HTV Return Vehicle (HRV) has been considered the initial model of the first manned re-entry capsule by Japan Aerospace Exploration Agency. The aerodynamic characteristics must be clarified for stable parachute deployment and landing because the blunt body is generally brought to dynamic instability at transonic and subsonic flows. In this study, free flight tests of an HRV shaped projectile were conducted in a ballistic range owned by JAXA Kakuda Space Center. The projectile motion during free flight was measured by the onboard four accelerometers. From the results, angular rate and angle time history were calculated. A method for constructing the motion model was developed, and we concluded that the projectile was in dynamic instability in some periods. Finally, a method for the rejection of the deceleration effect in free flight testing was proposed.