The type of black powder used can also make a big difference in fouling.
Back in the late 1800's, the W.R.A. Co., U.M.C. and others used black powders that produced soft fouling, allowing repeated shots without much reduction in accuracy.
Unlike today, there were a number of black powder manufacturers back in those days of yore, so the pressure was on to produce the best black powder available.
In Marlin's 1897 catalog, they list a number of Black Powders to be used in the .38-55:
American Powder Mills Rifle Cartridge No. 3
California Powder Works Pacific Rifle No. 1
Duponts Rifle FG
Hazards Kentucky Rifle FG
Kings Special FFG
Laflin & Rand s Orange Rifle Extra FFG
Oriental Powder Mills Western Sporting FFG
Oriental Powder Mills Wingshot No. 2
Today, the only black powder that is equal too and/or better than the good quality Sporting black powders of the late 1800's is made in Switzerland and burns with soft fouling in most applications. SWISS powder is available in the U.S. You can purchase it in 5# lots at:
http://www.powderinc.com/order.htm#swiss At $ 16.00/lb it is more expensive than Goex, but in my .44W.C.F. with a pitted barrel it still shoots accurately long after the accuracy with Goex has deteriorated significantly.
Have fun,
w30wcf