A method of modifying a projectile includes removing material from an aft end of the projectile, and coupling a device to the aft end. The method may be used to convert a spin-stabilized projectile into a fin stabilized projectile, by modifying the aft end of a spin-stabilized projectile to accept a fin kit. The modifying may involve removing material with lathe, and may include forming external threads on the aft end that may engage corresponding internal threads on the device, to effect the coupling of the device to the aft end. The modification method allows versatility in employing projectiles, including existing stocks of projectiles. In particular the method allows spin-stabilized projectiles to be converted to more accurate fin-stabilized projectiles.