Duplex loads in a rifle do help considerably with fouling. I found a good bullet with 4 square grease grooves handles the fouling quite well if blowing through the tube after each shot using straight BP. I use 3 breaths with a bore riding bullet. A grease cookie separated from the powder by a thin wad of bee brood foundation or wax paper, then a card wad and bullet allows at least 10 accurate shots without having to wipe the bore. Then all the fouling can be pushed out the bore with one DRY patch.
This was the late Paul Mathews method and it works to perfection.