Hey guys,
I have measured dozens of factory original, non altered American doubles that had a spot behind the chokes that got down to .018. Parker 20ga. guns on "O" frames that were ordered light weight had to ditch metal somewhere. These were original blue, original bore diameter guns in high condition. The Shooting Sportsman magazine Sept/Oct 2009 issue had an article regarding concentricity of Holland and Holland barrels and in the first 3 paragraphs addresses that question very clearly; industry standard is .020 3-10" behind the muzzle if I remember clearly. I have a copy scanned to my files if anyone wants to read it. email me at shachterj@aol.com
