There was someone who built a double rifle by sleeving barrels into a monobloc, but he used an interference fit. He pulled that off by dunking the new barrels' chamber ends into liquid nitrogen (to shrink them by contraction)sliding them into the monobloc, then letting their return to room temperature do the rest. I can't find the thread but it was fascinating. Avoided soldering.

It's possible, though, that shotgun barrels might be too thin for that to work.

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