The rust may be due the metal surface being absolutely clean. The oil, as well as putting up a protective barrier, will solve the problem of the clean surface.
This cylinder was cleaned in hot water and dried. It was then put under the grill to make sure the nooks and crannies had dried out. When I took it off the grill, still fairly warm, this rust had formed.
It can't be from the blackpowder residue so I can only think it was because of the perfectly clean surface that the metal was able to oxidize from the damp in the air.
When browning a barrel it works the same way. The chemical, Ammonium Chloride, cleans the surface off and allows it to oxidize.

https://i.postimg.cc/ZqMch2rV/Rust.jpg