I have a machinist friend counterbore the parent barrel, then turn the donor barrel down to a nice slip fit. Making sure that both the parent barrel and the donor are the same outside diameter, I then chamfer the mating surfaces slightly and run a bead of weld around the joint. After dressing down the joint and a little aging if is very difficult to find the joint. Of course such barrels should not be put back into firing use. It takes some time and work, but a good result can be obtained...
Ron