Abstract: The erosion characteristics of chromium-plated and nitrocarburized gun barrels were studied by erosion simulation test. It is shown that microcracks form in the brittle chromium plate when the chromium-plated barrel is exposed to high pressure and high temperature propellant gases. The cracks extend and end at interfaces between the coating and steel substrate. The propagation of the cracks can lead to brittle delamination of the chromium plate. In the nitrocarburized gun barrel, many wide and deep cracks form in the bore surface. The cracks penetrate through the nitrocarburized layer into the steel substrate, resulting in serious erosion of the steel substrate by combustion flow and causing the barrel failed. Based on experimental results, failure modes are proposed for erosion of gun barrels with the different surface treatments.