If its Lead fouling go pick up a copper Chore Boy scrub pad from your local store. Cut off a strip and wrap it around your cleaning brush. Spray in some solvent and run the chore boy wrapped brush through it a few times. It'll clean the lead fouling right up. If its copper or powder try some Bore foam. Spray it in, let it set for 10 -15 mins and run a patch through it. It'll come out blue if there were any copper fouling and black if powder. Repeat till the swab comes out clean.
Hope this helps.