It could be that rifling wasn't set in stone as it is now. The number of grooves, depth, width, twist, wall thickness and length were probably debatable and a smith with his own opinion made his own barrel. Maybe they thought that factory made was junk and only hand made would do for their customers. Barrel mills were cited for cutting corners and using inferior iron. Maybe too, making a pile of barrels was no big deal, the trick is to invite a few friendly gunsmiths over for a hammer in when business is slow, weld up a bunch of tubes, bore them, straighten them and ream them ready for the customer's desires. Each smith would go home with a dozen barrels maybe. When a customer wanted a certain bore, then the barrel was finished.