This scanned photo shows what it takes to do good work inside a double barrel. First photograph I have seen of a proper rig. I doubt if half a dozen gunsmith in the US are equipped as well.
Caption says it's set up for honing but no doubt used for all inside work. Key is the barrels are supported in a sliding table. Speculating, they are indicated inside muzzle and breech for true. Looks like the reamer shaft is bushed and can be extended past the muzzles were it can be indicated again for true . You could ream from the breech end but if so I can't figure how you could check it for run out. Perhaps the reamer has a hole in the end large enough for a dial indicator probe that can be reached from the muzzle.
Big difference from slipping a tapered bushing inside and cut by hand using a tap wrench.
William
Threaded Mode