I was repairing some broken rear window defroster strips recently using a small vial of copper powder suspended in clear lacquer sold for that purpose. The idea occurred to me, that if applied to a cast bullet, could a similar coating serve as a metallic paint patch or ablation shield? Copper and bronze powders are sold in hobby stores for craft projects and could be mixed with a small percentage of lacquer to act as a binder and adhesive. Has this been tried before?

I use it to bronze skulls and I would not advise as a patch. the lacquer/polymer binders would foul you weapons bore very quickly and possible raise pressures to dangerous levels depending on your loads

Well I don't know about copper/bronze but I do know that several companies sold lube's that were applied as paint to cast bullets in the past. Several are using it for commercial bullet sales now - Billy bullets here in Arizona uses Molly-Kote but others use polymer paint type lube
http://ammo.dynamic-arms.com/