Barrelmaker went to Europe to study how they made Damascus Barrels. A lot of the equipment was destroyed for its metals in WW I.
Those fancy patterned ones started as 5/16" square steel stock (which was originally bigger steel stock that was drawn down to 5/16"). Three were laid together and welded along with straight layers. Go to 10 min for explanation. They made them about 12" long and butt or scarf joint welded the sections together. They even show the twisting jig that twisted the workpiece.
https://youtu.be/kg6zBhbFrLQ?si=Ssd6iuVEOczG9qIF