Test firings of the 155mm XM825 artillery projectile have shown that its flight performance was affected by configurational changes to the base cavity. This was an unexpected result and a clear understanding of why these changes affected the flight behavior did not exist. A computational study has been made for the two different base cavity configurations which were flight tested. Flowfield computations have been performed at 0.8 < M < 1.5 and α = 1.0° using a recently developed 3D Navier-Stokes code. The computed results show the qualitative features of the base region flow field for the two base cavities. The base changes are found to alter the recirculation patterns in the wake which in turn affect the expansion at the base corner. These changes in the flow structure contribute to small changes in the base pressure. Aerodynamic force and moment coefficients have been obtained from the computed pressures and are presented as a function of Mach number. Computed results show small differences in normal force and pitching moment coefficients similar to that found in the range data.