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Gas curtain launch is an innovative method for underwater gun firing that enhances efficiency by creating a gas curtain. This gas curtain interacts with post-projectile gas and the surrounding water, resulting in a complex multiphase flow field at the muzzle, which significantly impacts projectile accuracy. To investigate the evolution of this flow field, a three-dimensional numerical model was developed, focusing on the distribution of shock waves, temperature, and pressure at the muzzle. The study's findings reveal that after ignition, the gas curtain passes through the projectile and exits the muzzle, forming an initial bottle-shaped shock wave through repeated expansion and compression actions. As the projectile penetrates this shock wave and the post-projectile gas continues to exit, a radially expanding bottle-shaped shock wave is formed. The Mach disk diameter decreases exponentially over time. The gas temperature within the shock wave bottle rises sharply at the Mach disk and remains high in the downstream region, gradually expanding while the overall temperature decreases. The high-pressure region is located downstream of the Mach disk and, due to the swirling effect caused by projectile penetration, this region moves radially along the Mach disk towards the downstream, with the average pressure gradually decreasing. [ABSTRACT FROM AUTHOR]