In this paper, we study the ablation phenomena associated with the operation of a capillary discharge for an electrothermal gun. Electrothermal-chemical (ETC) guns are used for enhancement of ignition and combustion of an energetic propellant. One of the major components of the ETC system is a plasma source based on a capillary discharge. In this paper, a model of the capillary discharge is developed. In this model, primary attention is paid to the ablation phenomenon. Different characteristic subregions near the ablated surface, namely, a space-charge sheath, a Knudsen layer, and a hydrodynamic layer, are considered. In this formulation, the ablation rate is determined by the parameters at the edge of the Knudsen layer. The kinetic approach is used to determine the parameters at the interface between the kinetic Knudsen layer and the hydrodynamic layer. Coupling the solution of the nonequilibrium Knudsen layer with the hydrodynamic layer provides a self-consistent solution for the ablation rate. According to the model predictions, the peak electron temperature is about 1.4 eV, the polyethylene surface temperature is about 700 K, and the pressure is about 10 MPa. It is found that the ablation rate increases with the capillary length. The ablated mass and the predicted total pressure agree with previous experimental observations.