The multilevel stepped-wall and rectangular observation chambers are designed to study the multipoint ignition process and the combustion stability control mechanism of the bulk-loaded liquid propellant gun. The expansion process and interaction of high-speed twin combustion-gas jets in liquid are studied by means of a high-speed digital camera system. The influence of the nozzle diameter, dual-orifice interval, jet pressure, and chamber structure on the jet expansion shape is discussed. The results indicate that a larger ratio of diameter-to-length can suppress the jet instability in stepped-wall chambers. Higher axial expansion velocity is found under the larger injection pressure, which it increases the instability of jet expansion process. Compared with a rectangular chamber, the axial expansion velocity is smaller, and the radial expansion velocity is larger in stepped-wall chambers under the same conditions. The theoretical studies of interaction of the gas jet with liquid were developed based on the experiment. Two-dimensional unsteady models are used to get the pressure, density, and velocity contours. The numerical simulation results coincide well with the experiment.