The previously developed mathematical model for macrobody ballistics inside an electrothermal accelerator was modified. The model can be used to predict the projectile velocity for given values of the mass and size of the projectile, the electrodynamic parameters of the capacitive energy storage, and accelerator parameters. The problem of determining the projectile velocity was solved in a hydrodynamic formulation by numerical integration of the equations of motion, the energy conservation equation, and the caloric equation of state. In the case of neglecting the dynamically changing friction coefficient, the calculation results differ considerably from the experimental data, but the model qualitatively describes the physical processes of interior ballistics of the electrothermal accelerator. Electrodes erosion influence was shown.