Keith, real wrought iron is available, but it's not likely to be sold in the form of a modern manufactured item. It's not so hard to find, reclaimed raw bar stock, most likely from an old bridge or building tear down, or maybe 1800's maritime chain links. There's a shade tree test that works pretty decent, make a partial cut in a bar, then try to break it with a sharp 90* hammer hit. Wrought will tend to show characteristic stringing.

I think antique wrought would be a decent example of what might be in a gun barrel. I'd have my doubts if a true simple steel was readily available to simulate what might have been around back when. Steve may have some wrought in his shop, and I wouldn't be surprised if he might not have tried an etch or two. While a too long etch on a barrel would not be a good idea, it may just be an exaggerated example of the differences in materials that it takes to show a color difference.