The brazed vent liner is a dome headed screw. What i do is use a 7/16" carbide end mill to remove the dome. Then i use a 1/4" end mill for about 1/8" deep, which is a start on removing the part of the vent liner below the dome. The next tool i use is a 9/64" drill all the way through the vent liner. Then i use a 82 degree 3/8" carbide spot drill to cut away what remains of the vent liner. Then i use a 5/32" drill through the remnant of the threads, and through the flame channel. This may or may not leave pieces of the vent liner in the threads. Then a 10-32 tap is used to clean up the threads.

The vent liner is extremely hard, and is hard on tools. It would be so very much better if Knight would sell these breech plugs without vent liners. Adding a vent liner to a plug without a vent liner is trivially easy, but removing the brazed vent liner can end up being quite costly.

Me, i am no machinist, so perhaps a real machinist could do the work more better.