Tom - I would not use a 15/16" Douglas barrel in .58.

Douglas used 12L14 which is leaded steel (fee machining), which is OK if thick enough and without any flaws. It is not gun barrel steel, although some companies make ML barerls from it. If the barrel was modern 'gun barrel' steel, I would use a 15/16" in .58. 1137 might be OK, but I'd rather have an inch across the flats or in diameter (at the rear 8" or so) for a .58.

A barrel that size would have to have a patent breech or drilled touch hole, not a drum and nipple or vent liner - too thin in my opinion.