What TOF says, or just use plain white vinegar, but don't slop any on the outsiude of the barrel, as it'll take off any finish.
Vinegar won't hurt iron /steel, but afterwards, flush it out and oil to prevent rust.
Vinegar can be left in the barrel indefinitely and won't harm the metal, but it does eat rust.
After soak of a day or a week whatever it takes, flush it out and use a jag and medium wire wool.
Used this method most on v bad barrels.
Good luck!