I've been told to use a brass sponge, like Choirboy or whatever is good for scrubbing pots and pans. It's brass, so it won't mar the rifling. I haven't done that--yet. Last time I had bad leading I wound up removing the jag from my cleaning rod, and used the brass rod by itself to scrape out the lead. Granted, it was a handgun, not a rifle.
I did just give in and order some brass dental picks from Midway, for the purpose of scraping out lead.
However, for some reason, the last time I scraped out the lead was a while ago, and it hasn't come back. At least not to that level. Perhaps it built up, due to a rough bore, and it's now not as likely? Maybe more often cleaning? Beats me. All I know is that the old advice of shooting FMJ/jacketed ammo to remove leading doesn't work (or so I've read): given how hard it's smeared into the lands I can believe that FMJ won't help at all.