Rust on an old 12-41 rifle

[Torpedo]

That is a really nice find!
I own one almost just like it, and as rust goes it's really not bad.
My approach is always to do as little as possible, simply shoot it, clean it and oil it as you would any other rifle.
You will find that after several sessions of this most of that rust will be gone.
The cleaning solvents and light oil will eventually get under the rust and lift it, don't be in a hurry.
As long as you keep it oiled and store it properly it will not get any worse and remember, the worst thing you can do to any machine is to let it sit unused.