The high temperature and pressure in the throat combined with the atmosphere created by the powder's burning causes an extremely thin layer to form on the surface of the barrel, in and just forward of the throat. This layer is like glass on top of mud (the more ductile material that it overlays). As pressure increases during firing, the base metal is able to expand and contract without cracking, but the much less elastic layer on top cannot. Minute cracks form, and from that point, the edges of the cracks become sites that are eroded by high velocity gas, and particles. As the process continues the surface will be divided into little sections of uncracked surface surrounded by circumferential and longitudinal cracks, that can become slightly tilted, sort of like the segments of a concrete highway. This causes a slight reduction of the inside diameter of the barrel in the area that it occurs. There is also an increase in the rate of jacket fouling that will require more frequent and laborious cleaning. At that point, a decision may be made to replace the barrel. For a given caliber, if powders that have lower peak flame temperatures are used, and the spacing between shots is carefully controlled, barrel life will be extended.l On the other hand, if you use a hot burning powder, very high pressure loads, and shoot long fast strings, allowing no time for cooling between shots, barrel life will be shortened. I believe that the reason that David Tubb reports longer barrel life by shooting bullets coated with a fine abrasive on a fixed schedule, throughout a barrel's life, is that when he does so, he is removing the glass hard layer in its very early stages, when it is only a few molecules thick, before it can cause problems. This is not to say that I would take this approach for benchrest competition, but there are many other applications where it might have merit.
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