An original Civil War pistol is very old. If it was made of steel, it was plain carbon steels, and the steels of that era were very inconsistent.
Until you study metal history you just don’t realize how primitive the process controls and how little they knew about steel and its chemistry.
What stresses, corrosive environment, your pistol has experienced in 150 years, you don’t know. It is a safe assumption that your pistol is not as strong as it was when it came out of the factory.
I would not load the thing to max. I would not load the thing at all. You are just going to add wear to something that cannot be replaced and there are lots of better made replicas on the market. And they are all worth less than an original.