It depends on what kind of flintlock action you want to convert the barrel to. Sometimes the barrel has to be shortened, and re-threaded for a correct breechplug that fits YOUR lock, and action design. For instance, T/C and several European designed Flintlocks have a " patent " breech, where the TH from the pan goes through the side of the threaded portion of the breechplug into flash channel, that leads to the middle of the back of the barrel, where a powder chamber, reduced in size compared to your bore diameter, has a hole from the flash channel into the powder chamber. The powder chamber is in the breechplug itself.
Others, more traditional side lock designs, have the TH enter the barrel IN front of the breech plug's face, and directly into the barrel where the powder charge is placed. The location of the TH in the two different styles of barrels can be quite different. For which kind of breechplug and flintlock are you trying to use the replacement barrel?
I converted a Percussion rifle to Flintlock years ago. The percussion gun used an outside Drum and nipple arrangement. The only problem we had was that the original builder of the percussion gun did not marry the breech plug and barrel correctly, leaving a gap, that caused us to have to plug the hole in the barrel for the drum, and re drill and tap a new hole for the new vent and liner when we converted the gun to flintlock.
The lock maker specifically made lockplates designed to be interchanged, so that the guns could be converted, if the owner chose to do so. That is the main reason I ordered that brand of percussion lock. So, you can have other problems with any conversion that only a close inspection by a competent builder will disclose.
