What eldeguello said.

If you ever get the opportunity to look at a worn out barrel, you'll notice that the wear starts from the throat at the chamber end and moves progressively forward. Since the bullet velocity, and therefore friction and force, are greatest at the muzzle, and yet the muzzle is the last part of a barrel to show wear, it obviously indicates that the bullet is not the culprit of barrel wear.

It's the very intense heat and pressure of the gasses at the throat that literally erodes the rifling away, much like water erodes rock over time. This is why it's important to keep your barrel cool when shooting. The hotter the barrel gets, the softer the steel, and the faster the gas can erode it away. Allowing the barrel to remain cooler by shooting only a few times (2-5 shots in a row depending on the cartridge, with higher capacity cases creating more heat and wear than their smaller bretheran) and then allowing the barrel to cool will greatly extend barrel life.

Mike