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Electrothermal guns utilize an ablation stabilized pulsed capillary arc discharge to generate a high-pressure propellant gas. The plastic propellant, usually polyethylene, is ablated from the capillary walls by absorption of the radiative flux from the arc discharge. Similar capillary discharges have also been used to ignite conventional gun propellants in electrothermal-chemical (ETC) guns. During the rising part of the current pulse, there are rapid changes in the chamber pressure and temperature as shocks propagate from the capillary and then reflect from the base of the projectile and back into the capillary. This causes redistribution of the current density within the capillary altering the spatial ablation rates with time. MACH2 is a general purpose MHD code that has been used to simulate these complex electrical, hydrodynamic and radiation interactions for a 12 mm bore diameter electrothermal gun configuration tested at Institut St. Louis (ISL). The simulation domain includes the capillary discharge, and an expansion chamber extending to the projectile. The radiation was adjusted to match the measured pressure pulse and total ablated mass. Large radial and temporal variations in the velocity, temperature, density and radiation fields were observed in the simulations during the rising part of the current pulse. These results suggest that one-dimensional models may neglect some important physical processes.