Push comes to shove, plug barrel, fill with KROIL, let stand 24 hours. Will actually penetrate between lead and steel, or between copper and steel. Will not etch a bore the way some cleaners will. Penetrating oil on steroids.
The shooting galleries did not give a crap about accuracy- but military armorers did use the mercury treatment. There was also an over the counter ointment used to kill body and pubic lice called Blue Ointment- used to contain mercury- and was used to de-lead many a barrel.