The after-oven-heating residue you described is quite normal. After it is cleaned to bare steel, the low oven heat mildly oxidizes the steel and leaves the residue you described. It's probably not leading, but powder and primer residue.
Barrel steel is porous, and firing residue is trapped in the steel. Regular cleaning only removes most of it. Repeated cleaning days after firing usually gets most of it out.
The military used to require daily cleaning of guns for three days after firing corrosive primed ammunition to get all the residue out.