Each old barrel got a Rem choke ream and tap at the muzzle end and external theads cut on the input end, with a sort of forcing cone cut on the input bore.
Then each of the barrels were screwed into each other end on end.
Old shotgun barrels go for $20 on ebay and are typically 30" long.
I cut off the chamber, extractor, lug, sight, etc.
I bought a Hastings Metro barrel, and looked at that before I started.