[snuffy]

Well everybody has danced around the real answer. The part or area of the barrel that causes the barrel to become inaccurate is the throat, also called the leade. It's that part directly in front of the chamber, a short taper from the outside diameter of the bullet to the bore diameter. In a .308 barrel, the outside diameter of the bullet, .308 has to be tapered down to .300, or the bore diameter.

I don't like to call it "wearing out" the barrel. Barrels that are "shot out" often have very nice sharp rifling ahead of the burned out throat. A trick that's often used by gunsmiths is to SET BACK a barrel. By that I mean shorten the barrel from the chamber end, by cutting about an inch off, then re-threading and re-chambering the breech area. This eliminates the burned out throat. They do this because the barrel when new was a very accurate tube.

The throat is burned by the brief extreme heat of the burning powder. Some say it's caused to some extent by abrasion of the powder granules. If you WANT to make a barrel go bad quickly, shoot a lot very fast without letting the barrel cool.

The tip rg1 made about cleaning, is spot on! More barrels have been ruined by improper cleaning than by shooting too fast. Cleaning from the breech with a good bore guide is the best way. If you must clean from the muzzle, use a good tapered brass guide. It centers the rod so it doesn't contact the rifling grooves at the all important crown of the muzzle. Damaged crowns are the number one reason for rifles shooting bad groups or causing flyers from an otherwise good group.