The appearance of the negative differential pressure(NDP) between the breech pressure and the shot base pressure in the interior ballistics indicates that a risk of the serious damage on the barrel or the ammunition has increased. From this reason, the gun system must be designed to reduce the NDP during the ignition stage. So, the effect of the propellant configuration has been investigated on the NDP of the interior ballistics. In order to simulate the interior ballistics with, the gas-solid flow, the developed two-dimensional axi-symmetric code has been used with, the Eulerian-Lagrangian approach and the LSHUS method. The propellant geometry with, the progressive surface area has increased the NDP.