In a gun system, polydisperse phenomenon may occur due to the local combustion by an igniter system during the firing process. The Eulerian–Eulerian approach still lacks the capability of describing particle mixing under given conditions. A detailed insight of the interior ballistics must be predicted for the better safety and the lower cost at the development stage. The multiphase particle in cell (MP-PIC) model based on the Eulerian–Lagrangian approach, known to be more efficient than the conventional Eulerian–Lagrangian approach, has been initially applied for the simulation of the interior ballistics. A good efficiency with the MP-PIC model has been obtained in terms of the computational cost. The axisymmetric numerical code with the MP-PIC model has been developed for two-dimensional analysis of the interior ballistics. As part of the verification process for the code, several test computations have been performed: sod shock tube, free piston motion problem, and virtual gun calculated by IBHVG2 code. The code has become reliable with well-agreed results with the comparison data. Additionally, a numerical model for the orifices to describe the vent holes of the igniter on the coarse grid has been developed with the lumped parameter method used in the IBHVG2. Based on the model, the pressure behavior in the gun chamber according to the igniter length has been investigated. The computational results have shown that the negative differential pressure occurs clearly when the igniter is sufficiently short.