Abstract: During the gun firing, the in-bore air column compressed by the projectile is ejected from the muzzle to the external ambience to form the precursor flow field, and has a great effect on the development of the subsequent propellant flow and the movement of the projectile even reduces the shooting accuracy. Thus, it is important to study this effect mechanism. The muzzle flow with a complicated projectile is simulated numerically by using second order AUSM+ scheme with MUSCL interpolation method. Multi-domain structured grids are employed to solve ALE governing equations. The real shooting conditions, for example, moving projectile, precursor and propellant flow, interior ballistics conditions are considered in the simulations. Two cases, with or without precursor flow field, are also predicted and the resulted shadowgraphs with precursor flow coincide with the experimental ones. And then flow patterns and development of the propellant flow field in these two cases are discussed in detail to reveal the effect of the precursor flow based on the numerical results. It is demonstrated that the precursor flow is the essential condition for the generation of the bow shock ahead of the projectile. And the maximal stagnation pressure in the area near the muzzle increases more than 2 times for the precursor flow relatively to the condition without it.