On rifles I build and many others you would not get the breech plug out with the liner installed as I adapt the face of the plug to fit the liner. Take it out, if it has screw driver slots or a allen face and measure the depth of the barrel wall then file it down. As Randy stated, knowing th type of rifle would help. If it gets real stubborn buy a replacement and take it out with an EZ out. They are not expensive. Your problem is one that can be remedied at your leisure, when you can take time to order a vent should you have to use an EZout, or what ever. It would be nice to know the threads also. Lots like Chambers White Lightening, and removal would be a good time to switch if that is your preference.
DP