A good friend of mine just gave me his Yugo SKS to sell as he needed the money.

I personally hand picked the rifle for him at AIM, I know how well he cleans his rifles, and  is almost as anal as I am.  I know what ammo he has shot in it - Barnaul. I know becuase I sold it to him.

Now, he cleans the rifle again before he gives it to me two weeks ago.  Standard Hoppes #9 for cleaner.   I put it in my safe with the other guns he gives me to sell, I take it to the show yesterday, lay it on the table and rust falls out of the barrel.  I break the rifle down and cannot see through the barrel.
I try to put a patch through it, it won't go more than an inch.  Finally after 20 minute I am able to force a .22cal brush through the barrel with a resulting pile of rust.  The barrel is completely rusted through.

The rifle was stored in a safe with the other guns he gave me and some of my guns.  No other gun had so much as a spot of rust on it, mine or his.

I sold it for $100, told the guy the barrel is total junk, let him look at it and he didn't care.  I figured it was a hell of a sale for due to its condition.

I know the barrels are not chromed in these, but damn, I have never seen anything like this. Any ideas?

I used to own a M91 in 7.62 X 54R that seemed to have a similar condition. No matter how much I cleaned the bore, it would always have some slight rust in the bore after sitting in the safe for a while.
This was back when all the available ammo for 7.62 X 54R was corrosive. ( Except for Norma, but no way I was paying $25+ for 20 rds of non-corrosive )
I used to clean with soap and hot H20, Hoppes #9, etc. I would even leave a coating of Hoppes Gun Grease in the bore after cleaning as well.