I'm asking for a friend who is trying to figure out how corrosion got in a barrel of a Winchester Model 70 (pre-1964 variant). 

Does anyone know if ammo for this old rifle is corrosive?

If milsurp ammo was shot thru it without the proper cleaning then it could be the cause of the rust.  Is there rust on the bolt face?  USGI WWII bore cleaner was able to neutralize corrosive salts.  It could also be that the bore wasn't oiled before the gun got put away many years ago.

I'm asking for a friend who is trying to figure out how corrosion got in a barrel of a Winchester Model 70 (pre-1964 variant). 

Does anyone know if ammo for this old rifle is corrosive?

Just curious what caliber is it chambered for?

The US transitioned from using corrosive primers to non corrosive around the mid 50’s. This is from memory as I am too tired to use my Google Fu powers right now. Probably even more likely if it is chambered in a military caliber. Being that it is a pre-64 it is highly likely someone shot corrosive ammo through it.

Does your friend live here in the islands? If the corrosion is bad then probably fired corrosive ammo and not cleaned it properly afterwards. If the corrosion is light/superficial then is it probably from the humidity.

Thanks for the replies.  What caliber... that was actually my first question too.  Let me get more info.

I bought a used pre64 M70 back in 1966 chambered in 270 Win.  Had no corrosion problems with it.

Thanks for the replies.  What caliber... that was actually my first question too.  Let me get more info.

...
Dala bet was 3006

I get choke old corrosive 3006.
I went to KHSC with friends and family to shoot off all that old ammo in my model 70.
After a while nobody wanted to shoot any more. It's hard on your shoulder.
It also had a misfire rate of approximately 10%.