Warning, this is a silly idea of mine so please feel free to call me a dangerous nutter.
Some years ago I had a .500 3" BP Jeffery double, the right barrel had a noticeable pit about half way down the chamber at about 2 o'clock viewed from the breech. I tolerated this for the life of the rifle while I was it's custodian. When a load seemed to get close to maximum the brass would acquire a small raised dimple at the pit location, I then backed off in my regulation loads. I have often wondered if a good clean of the pit, with Hydrochloric acid - which I used to use for stripping old barrel browning when it had gone beyond cleaning up and needed re browning.
My thought was a small deposit of dental amalgam ( mix of mercury, silver, tin and copper). This stuff is tough takes years of gnawing bones to break up fillings. I would be softer than the chamber steel so over filling could be honed / lapped out with little effect on the steel. It would be incompressible and hard. There may be more effective ways of rescuing a chamber in which extraction becomes difficult, but I have been out of gunsmithing for years. Amalgam doesnt seem very popular these days as there are many more painless dental filling materials these days, perhaps it may not even be used any more but some old near retired dentist may be persuaded to dust of his amalgamating equipment and try it on a pit?
All comments welcome.