Sorry, was a busy weekend and hard to get online. Thanks for the comments.
Turtle, if the barrel is a high carbon steel and hasn't already been blued, I don't see why it wouldn't work. Another technique I have used is to saturate some paper towels with a high vinegar solution (or mustard, I really like mustard) and then wrap a knife blade, you might have better success on a rifle barrel that way, but it would be hard to get a striped pattern. Maybe just some strips soaked in mustard and then wrapped? If you don't like it, you can take some steel wool and buff it out. If you try that, post some pics and let us see how it goes.