I think the culprit in the cone and just past the cone is lead.
The pellets are under great pressure, and leave a lead film almost like plating. Moisture trapped under and in along with the primer residue is what corrodes that area of the barrel.
I'm a big fan of a really stiff tight fitting bronze brush, rotary driven if necessary to get that lead out.
This crap about the barrel only being as dirty as the last shot is just that... crap. The lead builds up until removed. Easy to prove. Clean with just a Tico, or a pull through and put it away for a month. Now look just downstream from the forcing cone. Note the streaks of black. This is especially noticeable with a 410.
Yes, chrome is slicker and easier to clean. The lead does not stick to it as much.
The place plastic residue is found is in the choke cone.
