I wonder if the weld is there just to make the joint invisible. If the weld at the breech holds the tubes in place , would the weld around the joint need to penetrate the sleeve or just "fuse" the outer surfaces so it can be stuck up and re blacked ?
If the molten pool from the arc isn't too deep, it may not affect the tube beneath it too much.
Does that make sense ?
I realise there's heat present but it can be controlled and is localised so if the job is done slowly, maybe a pool at a time at one barrel, then the other, to allow for cooling, I'm pretty sure they wouldn't get too hot.
Another thought which leads me to think the weld is more cosmetic than structural is that they can't go all the way around, there's no room between the barrels between the top and bottom rib to work with.
I may be wrong but can't see a way to weld all the way around both tubes.
