Rust blue is what you want if you want anything at all. It is THE traditional shotgun blue. The fine rust formed during the first step in the process just breaks the polish of the metal, leaving a faintly satin sheen in the finsihed blue that won't be well matched by any other process. Brownells sells Pilkington rust bluing solution which I have used successfully on a number of small parts without having to build a steam cabinet. There was also a good article on the subject in American Gunsmith a year or so back, but I don't recall the issue specifics. I will have to try to find it and post the reference. Your main problem with shotgun barrels will be getting a big enough boiling tank if you are going to do this yourself? I use a Parkerizing tank.
Van's blue is a metal conversion process, like Parkerizing, and not just a selenium micro-crystal growing reduction reaction. I did an experiment once in which I treated the tips of several identical steel wire rods (saftey flag wires) with every cold blue I could lay my hands on (in my house, that is, which was about 8 different brands). I followed up by neutralizing the acids with Formula 409, then just rinsed them and left them exposed to air. This was winter time, so the humidity was below 50%.
After a week, all but the Van's had signs of rust. A month later, some were heavily rusted, but the Van's was still clean. Van's does not produce as deep a color as some of the others, but it is the best protective cold blue I tried. Brownells Oxpho-blue should be similar, but I didn't have any to try at the time. I also didn't have any Blue Wonder, which is a newer product with a degree of increased application complexity (pre-warm, pre-clean, apply bluing, apply developer) claiming to trade the lower convenience for overcoming the color depth and general shortcomings of the other cold blues. I have some now and will perform the same test to see what happens?
Nick