To explore the method of controlling the combustion stability of bulk-loaded propellant guns, the multi-structure cylindrical and cylindrical stepped-wall type observation chambers were designed to analyze the expansion processes of four combustion-gas jets by the high-speed digital recording system. The average axial displacements of the combustion-gas jet head at different times were obtained by processing the sequence diagrams of the jet expansion process. and the effects of A-type, B-type and G-type observation chambers with different expanding ratios and different jet pressures, nozzle diameters and nozzle center interval etc. parameters in A-type observation chamber on the expansion characteristics of four combustion-gas jet in liquid medium were compared. The experimental results show that the stepped-wall type structure can enhance the radial disturbance of the jet due to the induction of the step. When the expanding ratio ΔD/L increases from 0.3 to 0.6, the axial displacement of the jet decreases by 10.8%. With increasing the injection pressure, the gas jet intensity increases. When the injection pressure increases from 9.18 MPa to 15.30 MPa, jet axial displacement increases by 20.9% at 4 ms. When the nozzle diameter increases from 1.4 mm to 2.0 mm, the gas-liquid mixing is more intense, and the jet axial displacement increases by 24.1% at 5 ms. Increasing the nozzle center interval can make the radial expansion more fully and reduce the axial expansion process. When the nozzle center interval increases from 16 mm to 20 mm, the axial displacement reduces by 20.6% at 5 ms.