Without the rifle in front of me I hesitate to suggest anything else at this point other than to take it to a gunsmith and have him look the chamber over with a scope. I doubt the chamber was made that way because it would appear from the photo that the portion forward of the problem area is larger than the problem area. I am having problems seeing how a chamber reamer could do that during production. My guess is something happened well after production to booger the chamber.
OK, after some thought I will suggest just one thing. Get a section of brazing rod, not a skinny whippy piece either. File a chisel point on one end and poke around in the chamber. If it feels like there is some sort of built up area, then get at the edge of it and tap the rod with a hammer to see if you can pop it loose. The brazing rod if much softer than the steel so you will not scratch the chamber or gouge it. Don't go nuts with it. If this fails, get the gun to the smith.
"A Liberal is someone who doesn't care what you do, as long as it's mandatory". - Charles Krauthammer