The "worst-case" scenerio, is that you'll have to order a new breechplug from T/C, so (B 4 U do) match a drill bit to the root diameter of the threaded portion of the new/good nipple (or 209 seat), and drill out the remains of the old musket-cap nipple, leaving it's threads in the breechplug - which can later be either picked out or chased out with a tap.