He would certainly have needed to use a lathe to bore out the breech end to accept the turned down ends of the new tubes, and to turn the tubes themselves to the appropriate diameters and cut the step.
Unless the original breech ends were perfectly round and concentric at the join he probably left the outer diameter of the tubes at that end slightly oversized to allow for “ striking down” barrels so that breech ends flow smoothly into the new metal.
It is clear from the end result that there was a great deal of skill, and probably quite a bit of patience involved.
