As our gun erosion database increases for in-service and out-of-service 120-mm M256 tubes with M829Ax series rounds, distinct erosion patterns and mechanisms are emerging. Variability exists for M256 guns with M829Ax series rounds depending on round count, round type, round-conditioning temperature, and their order. Our M256/M829Ax gun system erosion model - with its interior ballistics, thermochemistry, and boundary layer components - is constantly being guided and refined by the erosion and materials analysis data from fired gun tubes. A recent refinement includes the improvement of the gun steel subsurface exposure model due to high quality, difficult to obtain data from in-service M256 tubes. Other recent refinements to the boundary layer heat transfer model are based on thermal data from M256 tubes. These refinements include the improvement/incorporation of case gas cooling effects, turbulent gas mixing/heating effects, and a very minor contribution from forcing cone-induced vena contracta cooling effects. These latter refinements are calibrated away from crack walls by positional thermal wall repacking depth, thermal wall transformation depth, and thermocouple data. A comprehensive gun erosion model is described for the 120-mm M256 gun with its M829Ax series rounds. In addition, a detailed shot-by-shot erosion modeling prediction is escribed for retired 120-mm M256 gun tube serial #1988. For this gun tube, the erosion prediction includes the two types of rounds fired, 829 and M829A2, and their three round-conditioning temperatures, hot, ambient, and cold. The gun erosion mechanism consists of heat checking the inert chromium plate, subsequent interfacial degradation of the subsurface gun steel substrate at the chromium crack bases, then chromium platelet spalling, and subsequent bare gun steel gas wash. This gun erosion model correctly calculates and predicts that the worst eroded region is at 1.2 to 2.4 meters from the rear face of the tube.