I think the best answer is: Avoid putting it there in the first place.
Not exactly helpful at this time, but it is easily done. Personally, I reload using Trail Boss powder with lead bullets and I don't get any more lead build up. At all. Like thousands of rounds later, there's still nothing except carbon.
The other way to avoid it is to use jacketed or plated bullets. These allow you to use any powder without concern of lead build-up. This is more expensive though.
When I found myself in your situation, with my blued GP100, I used a bronze bristled "tooth" brush and Ed's Red homemade cleaner. It takes some time and elbow grease, but it will come off. After doing this a couple times, though, I am completely sold on using Trail Boss Powder instead.
Remington UMC 158 LRN seems too shoot clean, if you shoot factory lead. But, honestly, if you're buying factory, jacketed are not much more money.