I once read some advice concerning old guns. I do not recall the source but the writer seemed well versed in his topic. I have come to adopt his method.
Start with a quite light load & work your way up. After each level of load examine the gun carefully, paying attention to the exterior. Look for any signs of "Soot" showing leakage through the wall. If that is encountered "Hang it on the Wall".
I would agree with Hal's load as a good ending point. If it passes that then I would use ordinary light loads & shoot & enjoy it. Do take extreme care in cleaning so those pits don't grow worse.
My personal opinion is that the Dangers involved due to dissimilar metals being joined by fusion welding has been Grossly Exaggerated. In the case of these barrels, the two metals do not just Butt up against each other but are fused together & this does not appear to me to be a major problem, or even a problem at all.
