Dueling pistols broadly lacked rifling but this English one had a unique hidden form of rifling called ‘French Rifling’. The idea was during a duel the pistols would be inspected for rifling to ensure neither side was cheating. But French rifling wouldn’t be seen, allowing one party an unfair advantage.
French rifling wouldn’t go the full length of the barrel so all but a thorough examination would suggest the gun was smooth bore.
Jonathan Ferguson from the English Royal Armouries Museum suggested that the name came from the English associating French people as being underhanded, rather than French usage of the rifling.
Royal Armories: 11:59 https://youtu.be/p1fYUafNpUg?si=
Johnathan mentioned in the video the rules of dueling would be agreed upon by the duelists. So a cheater might arrange for an opportunity to swap them or perhaps have a third party (that he bribed) pick them.