I bought a S&W m39-2 a couple years ago. Looking for a house pistol. The gun looked great but the bore was awful you couldn't see anything but black inside. Price was right so I bought it. I ran a bore brush and a couple of patches through it and the barrel looked brand new. Hoppe's #9 cleaned it all up. I think your "Pits" are just crap from it being shot and never cleaned. Pits would not cover that bad and would probably be around the gas port. Rust pits are generally small and round or roundish shaped. Could also be lead. Whatever it probably saved the barrel from being rusted. I also consider that most of these carbine were shot with GI ammo which was so cheap that it was ridiculous. Nobody but communists and Franceicon made corrosive and the chance that it was shot with that are not good. I know guys that NEVER clean a firearm they own, this may be one of them.