I have tried the old bath tub method, but the crud is solid. I'll try the .36 brush next, it may get the edges, but I can't see it getting the face, or bottom of the hole.
I have an idea of a scraper to fit the smaller hole. I have an old worn (patch retriever) that has shed it's barbs, I found that the diameter of the worn is the same as the hole diameter. Perhaps a tad smaller as I was able to clean some of the filth using the barbless worm as a patch jag. There's the beginning of a slot where the barbs were attached and, with a little dremel work, it can be opened up a bit for a thin piece of steel, (1/8th of an inch) to be used as a blade. Might not use a Dremel though, I think slow and steady might be better for this small work.
Not having done much soldering, the body of the worn is brass and the blade being steel is silver solder recommended?