The interior ballistics simulations in the single-stage gun and the AGARD gun are carried out, using solid/gas two-phase fluid dynamics code of two-dimensional axisymmetric calculation method. The calculation method is validated by the comparison of the results of the simulations in the single-stage gun with the experimental data and by the comparison of the calculations in the AGARD gun with the predicted data by the codes of some countries. In the present study, the effects of the placement of the igniter, the bore resistance and the projectile mass on the performance of the propulsion system are numerically investigated in the AGARD gun. Shortened and condensed igniter causes the formation of large pressure gradients in the propellant chamber at the ignition stage and strong negative pressure difference. From the simulations varying the bore resistance to the projectile, it is shown that there is the optimum value of bore resistance for bringing out the maximum acceleration performance in the gun system. The relations of the projectile mass to the projectile kinetic energy converted from the chemical energy of propellant and the maximum pressure in the chamber are also examined.