I saw the tool that did that rifling in St. Etienne. It was an ancient Cincinnati lathe with an attachment that drove a spindle about one turn forward, and then one turn backward through the barrel. How long the rifling was would depend on your cutter, but, the one I saw started just outside of the monobloc, and stopped just before where the choke would be. Perhaps 12-15 inches.
A lot of the specialty machines and tooling I saw was older, American equipment, designed or adapted for a single task. The press that puts the Darne name on the opening lever was built in the USA. Sometimes the punch broke and the guns had the name engraved in a recess, which is never as neat and clean as the punch, so few are like that.
Best,
Ted
