bung the muzzle up with a tight fitting bung ....stand upright in a bucket and fill from chamber with caustic soda mixed with warm water.....white vinegar is good but will take bluing off.....after a few days empty barrel and use a loose fitting brass cleaning bore brush with some strands off those pound shop stainless steel pan scubbers wrapped around and give it a good brushing.....then repeat with caustic soda etc.....