For lots of leading a 50/50 mix of hydrogen peroxide and white vinegar in the barrel will remove leading chemically. I use a vacuum plug from the auto store of the right size to plug the chamber, set the barrel vertical and pour in the mix. You need to watch the reaction and when it stops bubbling its time to pour out the solution and rinse with plain water as if left any longer it will attack the barrel and pit it. Its best not to leave it in the barrel for more than 10 to 15 minutes. Clean with patches afterward and oil.

The scum that floats to the top contains the lead and as such is toxic so dispose of where it won't contact food (ie. not down the kitchen sink). While all the cautions eem like a lot of trouble, it does save hours of scrubbing with a bore brush.