Sometimes the best approach is not the most practicable at the time.
Also, for all who read this there are 3 major types of blues-
Cold blues, which is a chemical usually applied at room temperature
Rust bluing, where the metal has a chemical applied to promote rusting, then the rust is carded off and the part boiled in water. This is done several times and pretty much gives the nicest looking blues.
Hot bluing, where the parts are suspended in a high temperature bath that usually has some sort of salts added to it to raise the temperature to around 400F. The main advantage of this method is how fast it is compared to the others.
Hot bluing is absolutely not recommended for double barrelled anythings that have soldered barrels! So that leaves cold and rust blues for doubles.
To make a boiling tank for rust bluing, purchase a section of roof gutter and 2 end caps. Use silicone caulk on the end caps and then secure them with sheet-metal screws so they absolutely will not come off. Costs very little. I have a nice stainless steel pan but it is too short for rifles, that I purchased at a hardware store catering to drywall installers. They call it a "mud pan". I found a larger one but they were all made out of plastic. If I could find a 30" SS mud pan I'd be in heaven! Anyway, I mention it in case they make such an item over on your side of the world.
Grumpy, I'm glad that I am not the only one who does not like the color of oxpho blue! Here is a pic of a hawken muzzle loader that I shortened for my eldest son last year, I used the 2 step method and it has held up well, even on the muzzle(sorry it's so big!) :
