Well if that's the case then the 'lubricant' would have to be the lead. Being that the lead is part of the brass alloy then that means it would be wearing therefore leaving deposits on the mating surface.
So if that's the case then when you are cleaning you are leading you barrel...
Hmmm interesting concept. I thought cleaning was to get the lead out of the barrel?
Actually, after a bit of research on the web, brass itself is NOT self lubricating. Those bearings etc you speak of are made of sintered brass (brass powder) where a dry lubricant has been added in the manufacture.