Sounds like you are saying the end of the compression tube sticks out from the walls of the compression tube. That would make the breech area hit the end of the barrel and try to pop the under lever back out. Not sure what holds the plug in the head end of the comp. tube, but maybe it has been "modified" for a tighter lock up. Machining the end of the tube flush won't help because it's the recess for the breech seal that's hitting the barrel.