The work presented investigates numerically and experimentally a formation of high velocity liquid projectiles in the course of the unsteady water acceleration by gaseous products of a propellant combustion. The projectiles were generated in a water launcher, a cylindrical enclosure entailed with a tapered converging nozzle. Previous studies demonstrated that liquid projectiles could be utilized as forming, microforming, welding, and boring tools. It is expected that other applications, such as detonation free explosive neutralization or emission-free coal combustion, are also possible. While the effectiveness of a water projectile is determined directly by its velocity, the principal constraint of the proposed technology is water pressure developed in the launcher. Numerical and experimental studies of the correlation between the water pressure and projectile velocity were performed. The work involved application of a computational fluid dynamics (CFD) package, strain gauge tests, and direct measurement of water projectile velocity. Several assumptions were made for the development of a numerical procedure. The behavior of propellant combustion products, at the pressure approaching 1 GPa, was approximated by the Noble–Abel equation, and the process is assumed to be adiabatic. Moreover, the formalism of the equilibrium thermodynamics is applied to a high pressure supersonic fluid flow. Recorded enclosure strains and projectile velocities confirmed the practical accuracy of the numerical method applied. The major finding of this study is the influence of traveling compression waves on water pressure developed in the barrel and projectile exit velocity. The high pressure, cyclically amplified by wave processes, is a significant engineering obstacle compromising durability of the system.