Otto-

You should cast those chambers with cerrosafe casting alloy (get it from brownells) and include the first inch or so of your bores/grooves.

Also, on that barnes x bullet idea...
I'd thought of doing the same thing with a 500/450 double rifle I got a while back. Don't go there. First make double darn sure what your bore/groove dimensions are then once you're sure of what you have go with hard cast or copper jacketed bullets.
With the barnes or other solid copper bullets you have a chance of screwing your barrels up. With jacketed or hard cast bullets you have little or no chance of (solely due to bullet choice) screwing those expensive barrels up with your bullets.

Another thing to consider is that your rifle was built to shoot *some* bullet, and it's not likely that bullet was solid copper or solid cupric alloy. Might as well start your load development with something that was likely to be available when the rifle was regulated.

--Tinker