This paper investigates the dispersion for unguided spinning projectile in order to explain the importance of the projectile production accuracy and the benefit of using guided projectiles. The total dispersion results mainly from the production inaccuracy of: (1) gun tube which can be shown in projectile's launching problems (muzzle angle, and muzzle pitching and yawing rates), (2) propellant which can be shown in projectile's muzzle velocity and muzzle spinning rate and projectile which is the projectile total mass, axial and lateral moments of inertia, and resultant center of gravity. The other causes are occurred due to free flight portion of projectile's trajectory which may be divided to: launching problems (vibration of launch tube), wind velocity and direction (wind profile). In this study, a trajectory calculation using a 6-DOF model was developed and applied for 155 mm M107 projectiles at different projectile and flight properties.