Abstract: In order to obtain the muzzle-leaving time of projectile accurately, a small caliber gun was taken as tested object, and four methods, such as high-speed photography, on-off target, photoelectric and recoil displacement methods were simultaneously used for the same projectile and a data acquisition system was used to acquire four signals, in order to search the best measurement method. Taken the result obtained by high-speed photography as time benchmark, four signals were unified in the same time coordinate. The results show that the high-speed photography is a direct and reliable method, on-off target is broken by high-speed burning gas flow before the projectile, and the powder gas triggers the photoelectric sensor earlier.