Receiver rust

[Ike]

Hi all,
I have had four different SKSs over the last few years. One has no corrosion and the other three have some receiver pitting. I know that SKSs can spend years and even decades in cosmoline and that there is always some surface corrosion on the receiver, but what level of corrosion and deep do the pits have to be before I should be worried?
What's the worst you have seen?
Cheers

 

[Curtton]

this is an interesting subject cos my sks i bot in the fall has minor rust under the barrel and under the receiver, which i removed by using a rotating brush in a drill but im wondering if the rust will return?