During in-bore projectile motion, considering the restitution coefficient of the collision between the former bourrelet and the bore as a constant couldn't really describe the projectile motion process. In fact, the restitution coefficient varies with the velocity of the former bourrelet hitting the bore. Through the finite element simulation on a single ball collision system of the projectile and tube's simplified model, the functional relation between the restitution coefficient and the radial velocity of the bourrelet was obtained. The relation was applied to the mathematical model of the projectile motion, the calculation result and the measured result showed the similar trend, so considering the change of the restitution coefficient has certain rationality.