I have no means to check the actual temperature with my set up but it's my understanding that there can be a diference of a few hundred degrees between some steels and iron. I think most who do this kind of welding would agree that there's a noticeable color difference between the two, although in my experience it's not as extreme as some say - I often hear guys describe the welding color of iron as "white", what I see is a good bright yellow. But when it comes down to it we'll all see the colors a little differently, as long as you can recognize the right conditions and achieve a sound weld that's all that matters.
Where's the fun if everything is easy - I agree 100%. Gun building wouldn't interest me near as much if it was easy. When I said it wasn't easy to weld thin material into a buttplate, it was more to offer an explanation for why it may not be seen too often on old guns.
A couple I have seen were of very thin material - anybody who has done even a little forge welding has seen how much material is lost to scaling at welding heat, so perhaps they were initially a bit thicker. But even so, some would have likely been thin enough to form cold. In the case of the buttplate with the folded joint I mentioned in my last post, I have wondered if it may have been shaped and folded cold, and then the joint welded and finished over a form / mandrel.
