Mostly to avoid localized heat distortion. Using either a Furnace or an Oxy/Act(rosebud) heats the full diameter of the barrels in the soldered area and it is allowed to cool slowly, so the metal stabilizes. Whereas TIG would only effect the inside radius of the top and bottom of the quarter section of the barrel. It would then have to put in my heat treat furnace and stabilized anyways. The other reason is because if for any reason at some point in the future should the barrels have to be taken apart...welds have to be ground apart as compared to solder that can be heated up and taken apart.

Colin