I haven't rust blued a barrel, but I have rust browned some. I cut a 4" black PVC pipe in half lengthwise on a table saw. screwed a couple wood feet on one side so it would sit level and put a couple thin wires across that section to lay the barrel on. In the bottom I placed toweling and made it a little wetter than moist. Set that on a shelf where the AZ sunshine could heat the pipe and place the other half of the pipe on top. I used a browning solution known to muzzleloader builders from Homer Dangler. I coated it in the morning, carded it off with a piece of scotchbrite or old blue jeans 5 or 6 hours later. Rinsed with distilled water and recoated the next day. After about 4 days of this the rifle had a nice even brown. If I had boiled that - it would have turned black just as any other rusting process. Thoroughly degrease before applying the rusting solution and more solution is NOT better as it will run and streak. I applied with a cotton dauber lightly so it doesn't run and form drops. The repeat coatings will even out nicely.
