If it's leaded up then try some of the Bore Tech Eliminator cleaning solution. If it's lead you'll be amazed at how dirty the patches will be till the barrel is cleaned of lead.
If it's corrosion you might try some stuff called Fluid Film. A lot of people use if for rust prevention on their vehicles in climates where a lot of salt is used on the roads in the winter. The descriptions/claims on the web site is that the Fluid Film, when sprayed onto a corroded surface will penetrate the rusty layers all the way to the metal and loosen the rust/corrosion. I know a guy who sprayed it on the wheels/lugs of a rusty trailer that needed new tires/rims and he was amazed at how all the lug nuts turned off easily without breaking a single one (he sprayed it a couple times over a three day period before attempting to loosen the lug nuts).
After that he started using it on rusty old guns he buys at gun shows and flea markets. Sprays it on, lets it sit a few days and then wipes if off with clean rags followed by one with Hoppe's#9 and then wipes the gun down with an oil rag after cleaning the barrel/internal parts.