The computation principle for the pipe-casing gun of the equal acress-section area is studied by applying the basic theory of compressible. The aerodynamic parameter of the pipecasing gun and the friction factor of rustless sted-pipes and copper-pipes of different diameters are measured. On the basis of this measurement, the outlet-velcity profile and the change of the rate and pressure on the axis are measured. The results show that the friction factor measured is reliable and the computation method is practical.