The barrel is said to be the most important part of the rifle as it is what actually has contact with the projectile. Mass produced barrels typical of the large rifle manufacturers are usually sourced not for the best quality rather the best price that a firearms manufacturer can purchase them for. The plan is to keep the prices of the finished product low and profit margin as high as possible.
This results in rifles that are more "hit or miss" when it comes to repeatable accuracy. Imperfections in the barrels bore will foul up sooner than other areas creating variations more noticeable from shot to shot than what a "match grade" barrel will produce.
Dan Lilja world famous maker of precision barrels has taken the time to produce an excellent short video that really says it all.