Not that I have xp with it, though I know a technique applied to cannons, wich could work just as well for muskets:
Have a nice sturdy brush, and a bucket of water, pour some water down the barrel, and begin scrubbing it, then pour the water out.
Of course the first time the water is very dirty. Just continue doing described steps, until the water is clean when you pour it out.
Then it's clean
Though I dont know if this is a good idea for cleaning muskets.