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The first known gun barrel thermochemical erosion modeling code is presented. This modeling code provides the necessary missmg element needed for developing a generalized gun barrel erosion modeling code that can provide analysis and design information that is unattainable by experiment alone. At the cunent stage of code development, single-shot comparisons can be made of either the same gun wall material for different rounds or different gun wall materials for the same round. This complex computer analysis is based on rigoroxis scientific thermochemical erosion considerations that have been validated in the reentry nosetip and rocket nozzle community over the last forty years. The 155-mm M203 Unicannon system example is used to illustrate the five module analyses for chromium and gun steel wall materials for the same round. The first two modules include the standard gun community interior ballistics (XNOVAKTC) and nonideal gas thermochemical equilibrium (BLAKE) codes. The last three modules, significandy modified for gun barrels, include the standard rocket community mass addition boundary layer (TDK/MABL), gas-wall chemistry (TDK/ODE), and wall material ablation conduction erosion (MACE) codes. These five module analyses provide recession, temperature, and heat fiux profiles for each material as a function of time and axial position. In addition, this output can be coupled to FEA cracking codes. At the peak heat load axial position, predicted single-shot thermochemical wall erosion showed unaacked gtin steel eroded by a factor of one hundred million more than uncracked chromium. For chromium plated gun steel, with its associated aack profile, it appears that gun steel ablation at the chromium cracks leaves unsupported chromixmi, which is subsequently removed by the high-speed gas flow.