FN49 in .30-06 was shot with corrosive ammo and not immediately cleaned. Bore looks crummy but the rifle is still as accurate as when bought nearly new in the late 1960s. Keep shooting.
Shot a 50 BMG M2 in the military. Gun was kept clean and well maintained but the tracers tracked like my tee shots: into the adjoining fairway. The barrel was worn out causing bullets (every 5th round was a tracer) to spiral, hook, and slice. Amazing sight. Time to replace the barrel.