ORIGINAL: Briman
but eating away of steel due to the temperatures of the high-pressure gases melting and carrying away barrel steel every time the gun is fired.
I've heard a different theory where the hot gasses harden a very thin layer of steel in the throat of the gun which makes the metal very brittle in comparison to the relatively flexible and resilient barrel metal. Successive firings cause the hardened metal to flacke or chip away.
I believe it is pretty well established that it is the "cutting torch" effect of high temperature, high velocity gases eroding the steel by melting it and carrying it away that "wears out" barrels. Undersized bullets magnifies this effect. For example, the M1911A1 of WWII vintage had .452" groove-diameter barrels, but GI hardball bullets miked .4490 to .450"". Barrel life of this combination was around 5,000 rounds or less, despite the fact that the pressures of this load were only running around 18,000 PSI. Examination of the shot-out barrels showed excessive gas cutting. When bullet diameter was increased to .452", this stopped!