Get your favorite copper solvant and plug up the end of your barrel. Fill your barrel up to the chamber and let is soak for a day or so. After a day empty the barrel into a container so you can use it again if needed. Now scrub your barrel with a brush. Follow that up with patches.

Repeat if necessary.

If your barrel fouls up that badly then you might want to consider fire lapping the barrel to smooth it out a bit.