If really grungy a bore brush and whatever is on the bench...W-D 40, CLP , brake cleaner, bore solvent , Marvel Mystry Oil , ATF , etc... which solvent/cleaner you use is not critical. There is usually just a build up of bullet lube and any lead in there is not baked onto the surface, it comes right out.
Just a little grungy a cloth patch on a slotted tip, I have even just pulled a rag strip , dampened with solvent, thru it. kind of like a makeshift Bore-snake.
A pencil and paper towel will work in a pinch.
Gary