Knight Breech Plugs...
The plug on the left is a DISC breech plug and the one on the right is a Mountaineer plug. You can see the Mnt plug is longer. Both of these are bare 209 plugs.
Which ever plug you have or get there is a good chance you may have to shim the plug with SS washer to fit the primer you choose to use. With either plug there is not real need of a Mag primer. The plug is built to possibly fit best with a W209 primer
This is a picture of a BP with a shim installed. The shim act as a head spacing device to keep the breech clean
If you need to shim I and other can help you with that.
The plug you are going to get with the rifle will likely be a FPJ plug like the one on the left.
That also means you will get a FPJ bolt housing that looks a lot like this. The piece supporting the nose of the bolt is a 'Primer Adapter' and that piece you will need to come up with. There is a guy on the internet that has the adapter and bare breech plug available.
This shows the primer adapter on adapter is a old Knight adapter the other is a Lehigh/Knight
This picture shows the primer adapted installed in a FPJ bolt...
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With this setup you just drop the primer into the open slot in the adapter. Closing the bolt installs the primer in the BP and opening and pulling back extracts it.
This picture shows the Vent Liner installed in the breech plug.
The other NFPJ Knight bolt (bare primer bolt) assembly originally for the Mountaineer series rifle but now being used in the DISC has primer rails built into the nose of the bolt and you need to slide the primer into the rails...
Hope some of this helps answer some questions....