The barrels are silver soldered together.This is normally done in a furnace with the length of the barrels protruding from it through a hole in the door.They can also be done with a torch as long as the heat goes properly through them allowing the solder to run properly through the joint.They must then be allowed to cool slowly as this will stop them developing any hard spots which can hinder with machining the lumps and chambering.
Welding is not really an option as it will only join the barrels on the edges of the joint and this wont be strong enough for use.Also because of the type of steel used to make barrel any welding will tend to create hard spots around the welds.
Hope this helps.

Regards

Mark