Look over my 16 bore single the next time you are in the shop, it is a conversion. Usually the the barrel, lock and often the buttstock were saved. An action, often Jones under lever style was fabricated with the fore thought of utilizing the existing parts. Labor was cheap back then. On my rifle it looks like a very small "patch" was silver soldered in place to seal what was the touch hole and to maintain a decent length barrel. Cool stuff for sure. One thing I have noticed it that many of the early conversions have very thin breech balls and often have a very sharp corner at the junction of the water table and the standing breech. I would like to take a deer wit my rifle net year, who knows how old the barrel is, love to see it do its job again someday.
Steve