I have an AKS-74 built from a Bulgy parts kit with a barrel from AK Builder which are chrome lined and made by them from Montana Rifleman blanks.

I had only shot 7n6 through it after building and have fired 400 or so rounds of it successfully; today I bought some 69gr Red Army Standard, and got a stuck case after 33rds, with the extractor slipping over the rim and proceeding to try and cycle the next round.

After tapping the case out, brushing and swabbing the chamber, I fired another 30-odd rounds and got a stuck case again, and proceeded to remove the case and keep shooting without cleaning; I experienced stuck cases every several rounds thereafter.

Has anyone else experienced this?

Should I buy a new extractor spring or is it an ammo/chamber problem?

I'd say go ahead and replace the extra for spring and maybe even the extractor since that would be the easiest fix.

Can you see any burrs or pits in the chamber?

Yes, there is some fine pitting close to the mouth of the chamber.

I'd suggest trying different ammo before you start replacing parts.

Try a different lot number and maybe some other brands and see if the issue follows.