You would do the job in similar fashion to the way described in the thread about cva breech plug modification. It won't be near as easy, because the brazed in vent is hard, which makes it difficult to drill compared to normal breech plug hardness. It can be done. I've done it three times. Non was easy. The work damaged and dulled tools.

It may be easier to modify an fpj plug to fit the Mountaineer, and install a vent whilst making the modification.