Corking the barrel, standing it upright, and filling it with mercury and letting it sit for a couple of days will remove it all!!

But since mercury is so poisonous, you may want a less toxic solution. I just wrap a full bore-size brass cleaning brush with medium-coursesteel wool, saturate it wih a boe cleaner known to remove metal fouling pretty well, and then merely scrub it out. Steel wool works alot faster than just a brass brush alone, yet will not harm a smooth shotgun bore...........