I'm sure lead and graphite have filled imperfections in the bores of used guns I've acquired. Shocking to get it all out finally and see the tool marks, scratches, etc. Of course it's those rough places that catch the stuff to begin with.

As far as residue leading to corrosion, from personal experience nothing matches plastic esp. steel shot wad material. Go an entire season without cleaning with the bores getting wet occasionally, if not chrome it's virtually a guarantee there's rust.