If you use soap and water to clean-- I can't think of any reason why not----- PLEASE, let the barrel sit with it full of soap and water for at least 1/2 hour, to let the soap do its thing. Don't RUSH cleaning.
The Black Powder is coated in Graphite to protect it from Static Electricity, to make it safer to handle. Graphite is a form of carbon. When the gun is fired, the residue consists of Carbon, nitrate salts, and Sulfur salts. Water dissolves the nitrate and sulfur salts. The Soap is needed to remove the carbon deposits, and GRAPHITE from the pores of the steel barrel. Let it have the time it needs to work.
Heating the water does NOT HELP it dissolve the salts. It doesn't do anything, except make you feel better! :shocked2: :youcrazy: Water is mother nature's " solvent". Heating the water doesn't make the soap work faster. Anyone who has had to wash in cold water knows that the soap is an alkalide, and chemical neutralizes acids. It also performs the chore of Emulsifying dirt, and carbon in particular, that is, soap helps the water hold the dirt in suspension, so that it is poured out with the water.
The best thing you can do to " Speed " cleaning of your gun is to use a bore Brush to scrape the crud out of the corners of the grooves that for the rifling.
I do this with soap and water in the barrel, and a damp cleaning patch on the brush. The bristles will poke through the cotton patch, to scrub the corners, and the cotton will provide a "vehicle" to which the crud will attach, and be pulled out of the barrel.
A bore brush is rarely need to clean a round bottom rifled gun barrel, as those barrels have no CORNERS. Similarly, a bore brush is rarely need to clean a chrome lined bore.
I share these latter tips simply to stress WHY I am recommending the use of a bore brush in your T/C, which doesn't have either round bottom rifling, or a chrome plated bore.
If you rush the cleaning job, you will continue to have gray colored patches come out the barrel even as you oil it for storage. The Gray color will be the graphite, that has to be pulled from the pores by allowing soap the time to get it done.
Can you just leave the graphite there? Yes, but you won't know if its ONLY graphite, or contains salts that will corrode the bore, until its too late. By using a good oil, or lube, you coat the barrel on the inside, which inhibits AIR from contacting the steel bore. Without air present, rust cannot occur. Corrosion from acids and salts still can, but not rust.