thats true with any barrel not just two part. Where the porting starts supposedly the effective barrel length stops, but at least some porting is always recommended as it reduces turbulance behind the ball when it comes out the end, therefore making it a little more accuratetbh mate progressive is proberly the best one part barrel I know of, one problem with two part barrels is that only the first bit of the barrel is effective thus so a 14" barrel may have only an effective length of 8"