The initial disturbance of a projectile emerging from the gun muzzle is a major reason for the projectile dispersion. Therefore, measurements of initial disturbances have a great value in improving the understanding. This paper establishes a mathematical model of the measurement system for the projectile's in-bore orientation, by utilizing the theory of ray tracing and kinematics. The layout of the optical apparatuses is attributed to an optimization problem, and the genetic algorithm is implemented to acquire an optimal layout scheme, which is further verified by test. Also, the linear timeinvariant characteristics of the measurement system are numerically verified in the context of spatial ray. These investigations will provide a technical support for the experiment, the optical apparatuses' layout and calibration of the measurement system.