Originally Posted by
ronlaughlin
Your second idea may work good. One could run the 7/16" end mill about a half inch deep. Then turn a stainless cylinder to be a tight fit in the breech plug. Drill through with the # 21 drill. Countersink, and tap the plug.
Or, tap the 7/16 hole, screw in a stainless bolt, cut off, grind, tap and thread vent liner. Bolt being ready made plug.