On close inspection with a magnifier I can see that the chamber has been sleeved. Obviously, I now know, but I didn't know chamber sleeving could be done. There is no obvious seam but I can see a slight difference in the direction of the tooling marks between the outside of the barrel and a concentric ring on the inner side. The ejectors also have been milled out and metal added so they are even with the rest of the chamber and there is a slight seam there.. It looks like a very professional job but it worries me that the gun is not in proof.
