Guys,
Since this thread started with the question of how were barrels cleaned originally, I might add that cleaning of a modern steel barrel is really quite a bit different than cleaning a wrought iron barrel. Steel will rust very much more easily than will wrought iron, there is a significant difference here. Wrought iron will stay bright when steel will look like an orange popsicle in no time!
Also, wrought iron is a physical mixture of pure iron and about 4 percent slag. This gives wrought iron a slightly porous surface structure. Lots of tiny places for black powder gunk to hide. I have found that to clean a barrel made from the original material, wrought iron, I must repeat cleaning, at least an oily patch wipe, daily for several days. There is no rust in this, but there will be additional black fouling on the patch. I clean until the patch come out with no fouling, then the next day's patch will again have additional fouling.
So, I believe that the original cleaning was not a one time effort, but had to be repeated several times even on different days to get a completely clean barrel.
Here is a microscopic photo of wrought iron so you can see why original cleaning techniques are different from our modern techniques.
Jim