Oils burn at different temperatures depending on their source, and composition. Not all oils are the same. With petroleum based oils, there are additives that do not normally burn well at the chamber temperatures created by Black Powder burning in your gun.
The result is a tar-like residue- not too different than the Tars left in pipes after burning some tobaccos-- that can only be removed by dissolving them with alcohol. Soap and Water only work if they sit in the barrel for many hours, and that can risk rusting the barrel under some conditions.
Hot, or "magnum" or " musket" percussion caps won't burn out this residue, either. The only way to avoid these tars is to not use petroleum based oils in the gun. If you use them for storage, then flush them out with a thorough soaking of alcohol before taking the gun to the range or field to shoot. To prevent rust during a short travel trip, simply put a bit of Olive Oil, or any vegetable oil on a cleaning patch and run the patch down the barrel to put a very THIN layer of oil on the lands and grooves. On humid days, I have been known to put the patch on my ramrod and run it down the barrel leaving the RR in place in the barrel, with the patch down at the breech end, inside my gun case, until I got to the range. I then highly recommend using several dry cleaning patches to remove all the oil possible from the barrel BEFORE pouring powder down for that first shot.
Some oil MAY remain in the pores of the metal inside the barrel. This often makes it much easier to load our PRB down the barrel, and keeps BP residue from Sticking against the walls of the bore after that first shot. That makes cleaning the barrel between shots much easier. I "Load to the mark" on my range rod, and having that clean bore allows me to run the PRB down the barrel to the mark much easier, while still giving me very good accuracy. The lube in the patch around the ball is enough to grease the bore as I seat the PRB in my .50. However, in larger bore rifles, I find that putting a greased cleaning patch in front of my loading jag, when I drive the PRB down to the powder puts more lube on the bore, in front of the PRB, so that there are no "bare" spots in the barrel where the grease in my patch did not lube the barrel. The greased cleaning patch comes out when I withdraw the loading rod. This is particularly helpful when shooting rifles, but also works in smoothbores, too.
When shooting BP rifles, the "secret" to achieving good accuracy is " consistency" in how you load, AND, how you clean, the gun each time. :thumbsup: