The effects of the friction between the projectile and launch-tube wall in a single-stage gun are examined by the interior ballistics simulations. The solid/gas two-phase flow code for two-dimensional axisymmetric calculation is used in the simulations. In this study, one-dimensional quasi-steady-state elastic relations are applied to the projectile in the estimation of the friction force. The calculation method is validated by the comparison of the simulated results with the experimental data such as the histories of the breech pressure and the projectile acceleration. The simulated results reveal that the impact of the shock waves to the base increases the friction force acting on the projectile. The large friction force in the tight fit case decelerates the projectile, consequently intensifying the chamber pressure. The muzzle velocity decreases with the increase of the projectile diameter, and this appears strongly in the low chamber pressure case.