you get better quality.the higher quality barrel usally is going to be held to stricter tolorances.alot are hand lapped,air gauged ect.alot of the custom barrels i've had fowl less and are easier to clean.these are a few reasons so many shooters spend the money on custom barrels.now there isn't anything wrong with a good factory barrel but most of the time you will see a big difference between the 2gun)