[michaelcj]

With Black powder or pyrodex water and some soap are the cleaning solvents to use. The corrosive salts and compounds in BP/P and especially in the percussion caps are NOT soluble in solvents but are Water Soluble.

Not sure what you can do now that the barrel is rusted [corrosion has already occurred] Suggest you try with hot water and dish soap and see if it helps at this point.

Normal cleaning:

dismount barrel from stock

remove nipple

Hot water and a bit of dish soap in a bucket.

with your jag on your rod, use a tight fitting patch

submerge the breech and nipple drum in the water

use the rod/jag/patch as a hydraulic piston sucking the water up into the barrel and forcing it back out through the nipple seat

In your case switch to your bore brush and see how much crud you can dislodge.

You might want to wrap some of a 0000 steel wool pad around a spare bore brush and see it that helps to remove some of the rust.

Back to the patch piston until the water you are pumping out is fairly clear.

Have the tea kettle on the boil and now rinse the barrel by pouring boiling water down the muzzle and out the nipple seat

WARNING!!! wear gloves or something to hold the barrel for this step it's going to get HOT!! and that's part of the idea as it will assist in the drying.

clean patches down the tube to dry.

and finish with a light coat of oil

Hope you can save the barrel but it sounds like the corrosion has got quite a head start.

Mike J