A special HVOF gun is used for aerodynamic research on internal flows of gas and particles in HVOF gun. The gun has rectangular cross-sectional area and has sidewalls of optical glass or transparent acrylic resin. Compressed air is used as process gas instead of combustion gas to visualize internal flow of the gun. The high-speed gas flows including shock waves in the gun are visualized by Schlieren technique. Particle trajectories in the gun are also visualized by high-speed digital video camera. The observation of erosion pattern created by particle collision on the barrel wall helps understand the particle trajectories throughout the barrel.