Much easier to use a 2" diam. section of PVC. Use caps on the ends, put the barrel in the tube, and fill it with boiling distilled water. The water can be reused for subsequent boilings until you get the color you want. You don't have to actually "boil" the part.
You don't need the fill spout in the middle. Just the tube and two caps will work. The barrel will come out covered in black fuzz. Card it off with fine, degreased, steel wool, reapply the bluing solution, and "boil" again until you get the depth of color you want. I usually do 5 or 6 applications. The color change is almost instantaneous...you don't have to leave it in the water more than 5 or 10 minutes at most. Small parts I boil in an old blue enamel pot.
After your final application and carding, kill the rust with a good oil. Watch it for a couple of days to make sure there is no after rusting. Rust blue is a very durable finish.