Lead oxide. Lots of cheaper 22 ammo that is old gets an oxide skin on it enough that i won’t reliable feed into the chamber of target pistols. Not all bullets oxidize like this but those using a harder alloy with zinc certainly will. Most casters shy away from zinc in their mix but commercial ammo not so. I doubt your tung oil is responsible but there are a lot of products on the shelf these days calling themself ‘ tung ‘ oil and most are formulated for ease of application. Todays users want faster drying, faster build up etc. so these products have lots of driers in the mix and maybe these chemicals are at play.
Rick
