Jack, years ago knew of a fellow that used mercury. Said there was nothing like it to remove lead fouling from pistol or rifle barrels. This is of course before all the poopla about the environment. Still you had to wear gloves and be very careful but he said that nothing did a better job.
By the way the mercury came from an obsolete vertical mercury boiler in a generating power station, supposidly the only one of it's kind.

David