I built an AR in 7.62x39 for a guy. The rifle consists of a J&S Surplus upper and High Standard lower. I know, it wouldn't have been my first choice either. Now the bolt does not cycle. The gas block is positioned correctly, but I'm going to check the gas flow today to be sure. The bolt cycles fine when manually charging it. After you fire it, you have to really pull to eject the case. I mean push the flash hider against something solid and use both hands. The brass does not look deformed after firing, no bulges, tears, etc. I've had some sticky cases in my rifles, but this is way worse. Does anyone know if the 7.62x39 rifles should have a different buffer spring? What diameter should the gas port in the barrel be? At this point, I'm thinking it's either a gas issue or possibly a chamber issue.
 

are there any markings on the case after firing?

i had a crap upper that had a really ruff chamber exibited about the same issue as you except mine would also tear a chunk out of the rim

shells came out like this... when they came out


corresponding markings in my chamber


I found a dowel slightly smaller than the chamber diameter cut a slit in it and wrapped a small piece of very fine emery cloth around it.

chuck in drill add some honing oil and make sure to keep drill moving in and out.

This is sort of the extreme end of it.

I started just with a chamber brush by hand.

many people will just use a shotgun or handgun mop with some flitz if it's not as bad as mine was.

It's not hard to overdo it with the emery cloth go slow and check often.

don't do this if your chamber looks relatively smooth and uniform it shouldn't be a mirror finish just uniform.

You may also have some gas issues