If it's truly leading and not lube from the bullet or powder residue, try the usual solvent on a patch followed by a brass brush., If that doesn't do it, take some Chore boy strands and wrap in several times around the brush and run it through the bore. It's made of copper so it won't harm the barrel but will pull the lead out and is cheap compared to the Lewis Lead Remover. You can find it nearly anywhere from the grocery store to Wal-Mart.
Chore Boy - Products
Chore Boy - Products