On the barrel that I had welded, an 1889 twist, The person who welded the barrel for me had me machine a tight fitting plug of copper to fit under the weld. After welding, it appeared that the bead and surrounding metal had the same hardness as the rest of the barrel. I was told that the plug was to keep the bead from entering the barrel, but did it also keep the welded area from becoming brittle by acting as a heat sink?