Actually you don't need any boiling tank. What you need is big boiling teapot with boiling water. Insert two rubber plugs from muzzle side and fill up both bores with boiling water and then to water all barrels with the same boiling water all over. Red and brown rust turns to black color immediatly after that.
Before this process barrels (allready rusted) have to be clean on carding wheel to get nice brown red patina.
For the newby I'd recomend to remember these several points as God Spell:
1. Use only clean water (distilled, deionized, spring)
2. Clean barrels before rust bluing as better as possible. The barrel surface must be 100% moisten-able after cleaning (Do not use modern cleaner powders or gels).
3. Try do not touch barrel surface during bluing even in rubber gloves.
4. Do not wipe barrel surface during bluing
5. Try to apply rusting reagent to barrels by one move of your hand only for each part of barrels from chambers area to muzzle (next time vice versa).
