canadian reloader is absolutely correct, if you look in the barrel with a bore scope you would be amazed or concerned just how imprecise, heat checked/cracked, wallowed out barrels can look and still shoot pretty darn good actually, it would be like looking at a cars well worn over heated pressure plate or disc brakes, if it wallows out it's usually at the muzzle end, also got remember barrel steels are not perfect you can get soft /hard spots in various sections of the barrel giving barrel makers fits. and yes you can wear/damage a barrel by improper/over cleaning.