| Aiming at the traditional internal trajectory structure of gun barrel, a new structure of gun barrel rifling with gradual cross-section structure is proposed. Taking the rifling of large caliber gun barrel as the research object, the paper calculates and analyzes the force change of the traditional rifle and the gradually changed rifle, include the squeeze resistance of projectile belt, the reverse side force of rifle, the frictional resistance of rifle and the section area of rifling grooves. The calculation results show that the gradually changed rifle is beneficial to enhance the ability of projectile belt to squeeze in, reduce the stress concentration at the root of the rifle, decrease the rifling impact and wear of projectile belt, improve the airtightness of projectile belt effectively. Meantime, the gradually changed rifle can effectively reduce the disturbance of projectile when it moves in the bore, so as to make the projectile run smoothly in the bore and reduce the movement of the muzzle. The gradually changed rifle not only ensures the shooting accuracy, but also improves the service life of barrel.