Wow... what is all that brown goo in there?
Looks like all that's left is the chamber plug. Put an empty 12g in the front just like if you planned on gassing it up. Then slide a 1/2" or so wood dowel down behind it. A couple good taps with a hammer on the dowel should pop that chamber plug loose. Might want to mark the dowel at the edge of the gun before you start beating on it. The 12g wont all the way through... only about 3/4" or so (it bottoms out on the permanent sleeve in there). Once it moves a bit, pull the dowel and 12g. You might need a thin dowel or rod to push the chamber plug the rest of the way out.
Man, that is a touchy one though. It looks like someone actually glued the chamber plug in. A bit of heat might help (boiling it) but you can't heat that area up too much with a torch or you'll end up ripping the whole "permanent" sleeve out of there.