Personally, I do not like the idea of letting a barrel sit for days in some strong chemical bath. I'm not sure what would happen if you placed it in simple soap and water. Perhaps the fact that the barrel is submerged, it would not get enough oxygen to rust. I just clean them.

Perhaps you could make up a chamber as you describe, take a chunk of steel, degrease it, and clean it off with alcohol, then put some powder piles on it, ignite it, then test your bath to see if the steel will rust.

Sounds like you are really pressed for time and your rifle, these rifles can not be ignored. You ignore them and they will rust on you. Good luck with your experiment.