I was at one of them merchandise stores that sell a little of everything cheap... And they had nylon and brass brushes there, 3 for a $1.00 so I got three of each.
What I do is take the barrel out of the stock. Take a tooth pick and put it in the nipple hole. Then I take some "Soft Scrub" (that soft paste kitchen scrubber stuff) on one of the nylon brushes, and scrub around the nipple and bolster area. If it gets real stuborn then I change over to a brass one. After I get all the fowling off, I wipe the area off with denatured alcohol. Now with everything clean, I pull the tooth pick. Then I soap water the rifle barrel as normal, and clean the rifle.