My experience indicates it is more myth than reality. I have found all you end up doing is ironing the lead into the rifling. Doesn't make it harder to remove but doesn't clean it out either. Just take a copper brush wrapped with a few strands of copper Chor Boy and run the brush up and down the barrel a couple of times. Cuts the lead out quickly and cleanly. Usw 100% copper Chor boy though. Do not use the copper coated steel wool type.
Take Care
Bob
Take Care
Bob