As the test gas in a gun tunnel is compressed and heated (non-isentropically) by a light, fast-moving piston, first consideration is the optimum piston weight. Although various aspects of the influence of piston weight on gun tunnel performance have been studied, it is not possible to decide from the existing literature what piston weight is required for various conditions. The technique whereby the piston is rapidly brought to rest at the end of the gun tunnel barrel, and the pressure peak which results is equal in magnitude to the final equilibrium pressure, is called the equilibrium piston technique. Then, the equilibrium piston technique has been developed to estimate equilibrium piston ma but this technique cannot give an appropriate estimate for the piston weight. Computational modeling of gun tunnel operation can enhance the understanding of important ?ow processes and assist in the identi?cation of the ?ow conditions that are produced by such facilities. Such modeling can also be used as an aid in the identi?cation of new operating conditions or the design of new or modi?ed facilities. In the present work, the performances of the various shock and gun tunnels that are built in all over the world and are being used are deeply studied in order to localization of knowledge of design of such short duration impulse facilities. For instance, the 2 in. and 3 in. gun tunnels are described and the performance of the 3 in. gun tunnel is investigated numerically. After investigation of the performance curve of the facility, effect of weight and shape of piston are studied. Pistons in different shapes and weights are investigated for the same initial condition to optimize the gun tunnel performance with optimization of running time of wind tunnel. Numerical simulation in present work is in very good agreement with experimental results. Significant influence of the existence of a piston is shown by comparing the gun tunnel results with results of a shock tunnel in the same initial condition and around 250% increase in running time is gained. Numerical results show that equilibrium piston technique is not a good way to estimate suitable piston weight and there will be a lighter piston which can increase run time of the gun tunnel around 60%. Then a modified technique for the operation of the gun tunnel is suggested based on numerical results. If the piston mass and the initial barrel pressure are chosen correctly, then the peak pressures associated with the gun tunnel will be eliminated. Under these conditions the piston is brought to rest with no over swing. Moreover, investigation of the shape of the piston shows that piston shape has no significant effect on the performance of the gun tunnel. Finally effect of imperfection in initial diaphragm rupture as a significant loss in pressure of reservoir in end of barrel of gun tunnel was numerically investigated. The results reveal that imperfect rupture of initial diaphragm significantly reduces the shock wave strength, shock wave speed and equilibrium pressure of reservoir. At a diaphragm pressure ration of 50, the equilibrium pressure decreases by 13% when diaphragm is 90% open and 27% when diaphragm is 80% open. Keywords: Gun tunnel, Shock tunnel, Hypersonic flow, Shock wave, Piston, Diaphrag