All good stuff. If welding outside pits in the thin areas of the barrel think about turning a close fitting copper or brass plug to fit the bore. This will be a heatsink when welding the O-too-thin areas and keep you from melting the wall into a hole. Some cold hammering on the welds with the plug inside also cold forges the grain structure a bit and hardens/toughens the weld. This is usually not necessary in the thin areas since the pressures are low there. Just more confidence factor.