The paper aims to apply the dynamic gun theory to establish a mathematical model to determine the force of the shot for two models of 5.56 mm assault rifles with different barrel lengths (460 mm—model A and 380 mm—model B) when firing 5.56 × 45 mm NATO ammo. The research results indicate that the force shot mainly depends on the pressure in the barrel, and when the bullet exits the muzzle barrel, the force of the shot is different for two model rifles. The force of the shot for model A is greater than that of model B when the bullet leaves the muzzle barrel. The mathematical model in this paper is a reference for analyzing forces acting on the mount and the stability mode of this gun.