If the stock is pinned, use pin punches to remove the pins. That will get the barrel out of the stock. Expect that breechplug to have a smaller powder chamber in it- possibly as small as .25-.33" Try using a .22 caliber bore brush, or a .30 caliber bore brush to see which size fits that chamber. You can wrap a cleaning patch over the bore brush to clean the powder chamber.
Remove the nipple, and use a pipe cleaner, or at least a bent paper clip to scrap the crud out of the flash channel, that runs from the nipple to the middle of the back of the barrel, and that powder chamber. If you can find a gunsmith to take that breechplug off, you can have him use an end mill or drill bit to open up that powder chamber to a larger size- possibly as large as the bore diameter. That will allow you to clean the chamber area with the same jag and patches you use to clean the rifled portion of the barrel.
The Flash channel may also be way too small. That can be usually drilled open a bit, without harming the integrity of the design. If you don't open it up to a larger size, at least get in there with emery cloth and polish the heck out of that channel. A hand drill, a thin dowel, with a slit cut in one end to hold a strip of emery cloth will let you polish the channel at high speed, and get it done quickly. The smoother it is, the less likely crud will form and stay there.
Oh, if you are using any kind of petroleum based oils, or lubes, STOP. The gum up the works. Use only vegetable based oils and lubes in MLers. Its one of the " Great Secrets". Much of the crud problems people have with MLers, using both Black Powder, and the Synthetics, is the fact that oil and these powders produce some serious gunk that glues its self in the barrel. Soap and Water will dissolve most of it, and alchol will dissolve the rest. But, that is not much peace of mind when you are hunting. :hmm: :hatsoff: