I agree with Steve regarding pattern welded barrels; nor would I lengthen the chambers of fluid steel light weight British or Continental game guns.
U.S. doubles tend to have thick chamber walls, but I also would not recommend lengthening a chamber is the pre-lengthening end-of-chamber-wall-thickness is < .110".

There was a statement by a British barrel specialist who observed some difficulty in cutting the chokes in damascus barrels related to delamination. It may have been David Dryhurst who made the damascus barrels for Greener using vintage tubes 2007-2008.