Hi Greg,
When you look into the powder chamber from the top with the screw plug removed, you will see that the opening into the threaded action is already a substantial radius. On my gun that radius clears the entire vent liner cone and then a little more. On your gun that radius would probably also clear your vent hole even if it was drilled straight in. Simply by filing the notch on the screw plug a little deeper and maybe giving it a radius and then drilling your vent hole angled a little forward you should be OK. If you install a white lightning vent liner the inside cone should clear the plug if you file the notch a bit deeper. The problem that you may encounter with the vent so close to the face of the plug, if you get much fouling on the upper threads of the screw plug, some may slough off and block the vent when you open the breech. I believe your WL liner will not reach all the way through the barrel wall to the plug. Obviously, you will want to check that and may have to file off a few threads on the liner before installing but I don't think you will have to. As you know, the barrel wall at the breech is thick so a simple vent hole could result in slow ignition after a few shots. Clearly, Durs Egg resolved these issues on the pair of guns he made one of which is the gun that Ferguson used in his demonstrations to ordnance and the King. However, on the existing ordnance guns, the lock is positioned forward as mine is and not as it is on your stock.
dave
