Severe Lead Build Up

[AJ]

Fitst off get rid of the nylon brush. Get a good heavy duty bronze brush to do the cleaning. If its lead build up, the brush will do the job but it will take some elbow grease.

For plastic build up, there are a few chemicals that will aid in the removal. Slip 2000 has Gas Piston Parts / Choke Tube Cleaner that eats plastic fairly well. It won't disolve all of it, but it will soften it to allow fairly easy removal with a bronze brush.

[AJ]

Another thing that works very well is the Outers Foul Out III. Its an electronic system that using a plating process to remove copper or lead fouling from the bore. They work extremely well, but are a tad on the expensive side.

[carbonhunter]

try and bronze brush/cleaning rod attached to a drill, i do this to clean the forcing cones on my guns alot, as they get super super dirty after skeet league

[2500HD]

We used to take our old smooth bores and soak the barrels in pan of diesel fuel over night and the lead would come out very easily. I'm sure today many of the gun scrubber chemicals will work too just more $$$.