I've swamped three rifle barrels and one pistol barrel. I used a quick blue on the barrels, and then scribed guidelines on the sides of the first four opposite flats. I then used a grinder to grind the flats close to my lines, stopping frequently to cool the barrel with water. Once down close, I filed the barrels to the line, then blued those four flats, scribed my guidelines, and ground and filed the last four flats. You can do a barrel in a long day. Lots of work, and only to be attempted if you simply can't find what you need.
All four barrels where GM barrels. All the barrels have shot just fine. The trick seems to be to work on opposing flats, and keep the barrel cool.