Two stage light gas guns, which were small, middle and large size, were installed at Hypervelocity Impact Test Facility Laboratory of Spacecraft Environment Interaction Engineering Kyushu Institute of Technology to conduct space debris impact test. The large one achieved a maximum muzzle velocity of 6.2km/s and the middle one achieved the maximum muzzle velocity of 7.9km/s, whereas the operational muzzle velocity of the small one remained 4km/s. However, operation at high muzzle velocity area of the small two-stage light gas gun was required in order to simulate space debris impact in low earth orbit and to realize a counter-fire with over 10km/s of impact velocity. Therefore, we carried out the muzzle velocity improvement of small two-stage light gas gun with a goal of over 7km/s.