Try Hoppes lead remover. Swab it back and forth a few times and let it soak overnight. Cut a few strands from a copper 'ChoreBoy' scouring pad. Wrap it around the swab and stroke it through a few times. The copper will cut the softened lead but will not hurt the barrel. Repeat as necessary. If it's really leaded, it may take a few overnight soaks.

There are some copper plated steel scouring pads that WILL scratch the barrel. Make sure its the copper ChoreBoy.