Rust isnt a virus, it doesnt just keep growing. If you block access to the metal for water and oxygen, no new rust, even if there is rust already there.
If that were true, all you would have to do to stop rust from spreading on your car is to knock the loose rust off, & paint over top of the remaining rust. Go ask a car paint pro specialist (not Econo or one of the other budget shops) why they bother with the time & expense of sand blasting all rust off a car before repainting.
Rust spreads on a molecular level as electrons transfer from iron molecules to the surrounding oxygen molecules, changing the makeup of the iron and turning it into rust.
For the layman, this means that the free oxygen molecules already trapped by rust can transfer to other atoms of iron and create more rust, even under a protective layer of oil or paint. This used to be taught in high school chemistry class btw...
Look at blue steel, i.e black rust. Surface is already rusted and would rust (red rust) badly very quickly were it not oiled. When oiled, no additional rust
You're thinking of cold-blued steel which offers no protection from rusting. Hot-blued steel is a man made process of artificially creating 'black rust' in a very controlled way. Black rust is the only form of rust that naturally hinders further oxidation, which is the reason it became a popular finish on firearms. It does not contain free oxygen molecules that can spread under oil. However, like all rust, it is porous which can allow oxygen to get in which is why regular oiling is recommended. Hot-bluing changes the surface molecules of steel to black rust - cold-bluing 'permanently' dyes the surface of steel to mimic the look of hot-bluing without the use of the caustic chemicals, however, because it's just a layer of dye it has no protective properties. Technically speaking: 'black rust' is formed through natural processes where blued steel is a man-made process of artificially recreating black rust (or the look of black rust in the case of cold bluing) in a very controlled manner.
Oil is a great barrier, but it is also a fluid, and with a few exceptions, remains a fluid. Over time, oil can run off leaving spots unprotected where fresh oxygen can now cause the rust to spread, even under the parts that are still protected by a layer of oil. I have a huge steel work table (3ft x 6ft) that was once my step dad's. He used to oil it about every 4 months to keep rust away. 6 months after he died (probably 8-10 months after the last oiling), there were spots of rust on the table top where the oil had flowed from the high spots to the low spots leaving the high spots exposed to oxygen & moisture, but even on the low spots that still had oil, there were trails of rust.
Take a look at part of a 22 rifle. The bolt was removed, cleaned, and protected with Break Free CLP after every time it was taken out & shot. However, the last time was about 2 years ago. The blued action(or receiver as some call it) and barrel of the rifle have never been oiled since I bought the rifle back in 2008. This is kept inside an air conditioned house so the humidity is MUCH lower than out in my wood shop. Despite having a protective layer of CLP on the bolt, and despite the action & barrel having never been oiled, there's more rust on the bolt, than on the action or barrel.
As you can see, the bolt clearly has a thin layer of rust over almost the entire exposed surface where the CLP has run off over the last 2 years, but there's only a couple flakes of rust on the blued steel which again has never been oiled. Where hot-blued steel WILL rapidly rust is if you leave the oils from your hands, or it gets rained on and you don't wipe it down, or leave it out in a very humid environment, but will still rust slower than bare steel in same environment. Cold process blued steel on the other hand will rust very easily because it's only mimicking the look of hot blued steel.
Here, you can see where runoff has occured on the blued steel part of the bolt. The top part appears matte / dull where the CLP has completely runoff after 2 years, then you have the shiny area below where CLP is still there, but no rust anywhere on the blued part of the bolt.
Here is an enhanced version that more clearly shows the line between oiled surface & where the oil has run off.
One could almost paraphrase Dr. Ian Malcom that like "life" (or a virus) "rust, uh, finds a way"
Especially when I don't have time to oil everything down regularly.
Anyway, time to strip down the rifle, clean it up, & give Frog Lube a try.