Tom,

unfortunately, it depends on the steel and its heat treat. The rupture strength of a gun barrel depends on the pressure of the load, the inner diameter (.58 in your case), the ultimate strength of the steel, and the wall thickness.

If you have or can get the Rockwell C hardness of the barrel, you can determine the ultimate strength of its steel. I'm not sure what pressure black powder loads can get up to, but someone should know that. Then a bit of engineering 201 and you have an estimate of the capability of your barrel.