I use a 50/50 mix of hydrogen peroxide and white vinegar. Seal one end of barrel (electrical tape, cork, whatever), pour mix in and let stand 5-10 minutes (I always leave it 10). Works great. If you REALLY have ALOT of leading (as I did once), it may take more than one application. It is very effective and the price is right.
Pour it in a dry bore, no oil. If you've already got oil or solevent, run 2 or 3 dry patches through first.
When it's working it will look like a grey milk shake type foam is bubbling up.
When you've got the lead out, just clean and oil as you normally would.