Firing fmjs through it will make it worse , heat and pressure got it in there's and more heat and pressure will pack it even tighter... A bronze brush and some sort of lead cleaning fluid will get it out... It will definitely take some scrubbing.... Try loading not as hot and a better cleaning routine , cleaning more often... You will get it out eventually...Then take measures to keep it from building up again... Loading lead hot will make leading happen bad... It's honestly why I stay away from it anymore if possible... As mentioned above coating your bullets will greatly reduce leading... There's some good info on coating on you tube....