The chamber volume inside projectile is a crucial parameter affecting the interior and exterior ballistic performance. Conventional methods have disadvantages of inefficiency and inaccuracy. An ultrasonic measurement system based on the size of the warheads appearance was designed to deal with the volumetric issue. First, the wall thickness of the shell in an arbitrary position was tested by the linear movement of ultrasound probe and the axis rotation of the projectile body. Then all parts of the section round and diameters inside the projectile were fitted and calculated according to the shape of the known geometry sizes, and at the same time the indoor interval lengths were calculated too by subsection discriminating method. Finally, the chamber volume was calculated and error was analyzed. Experiment results show that all of the parameters of the chamber volume of projectile could be tested quickly during one test progress, and it is easy to realize automatic measurement.