Phosphor bronze brush and a lot of elbow grease. Surprising that you have lead fouling though - mine never gets lead fouling - what have you been firing - what load?
There is an alternative to the brush. Birchwood Casey Lead Remover Cloth. It's really intended to remove the blackening you get on the face of nitro revolver cylinders, but I have successfully used it on barrels. It is a bit of a faff to wrap the correct thickness on a jag, and will need to be re-wrapped frequently to maintain a tight fit to the barrel. Don't worry that it has gone black - it actually works better when it is black.
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