You said surplus ammo, but not what country. May be unrelated. I am just curious as I got some portugese .308 off a friend that was HOT! he had to quit using it because it was so hard to extract after firing. Ran great in my Cetme, big flames out the muzzle brake, super loud, chunked the brass about 25yds.
If I were you, I would clean the chamber good, clean the channels/grooves in the receiver, disassemble the bolt, clean the internals of the bolt and oil it. You could have a metal burr somewhere on/in the bolt since it hasn't seen allot of use.
I own firearms not to fight against my government, but to ensure I will not have to.