Doesn't Ed Rayl use 4140? I know his barrels are a real bear to file, dovetail, engrave or whatever. I was told many years ago by him directly that he was using some 4xxx type of steel, always thought it was 4140.
The iron barrels Brad was selling on occasion were made from round stock taken from an old bridge dismantle, if I recall correctly. I believe Bob Hoyt was boring, rifling and profiling them. Ed Rayl has also done this with customer-supplied iron rounds, afaik. I still have a bunch of those round bars, very little corrosion at all on the exterior despite being 100+ years old now.