I used 1/2" cold rolled key stock, followed John's recommendations to the letter, and came up with some very useful jigs.
When you C clamp the rails to the barrel, they conform to the shape of the barrel easily. And they do not take much of a set, even with holes all along their length.
Now I get Dave Rase, Mark Weder, or Mark Wheland to inlet my barrels, and cut the ramrod channel and hole. That's about eight hours of work you get done better for about 100 clams...worth every nickel to me. But a person should do a few the hard way first, so that they appreciates their service.