What would make an auto AK not function properly in semi auto only?

[bruh44]

I’ve got a Valmet M-78 in 7.62x39 that I just got back with a new barrel on it. Auto is great. It will fire one, maybe two rounds. The next gets locked in the chamber with a light strike. The chamber is very difficult to open when this happens. Is this a worn out fire control group? Springs? Any other ideas?

 

[saxon]

almost sounds like hammer follow...

 

[sniperdoc]

Although highly unlikely in a Valmet, I had a Maadi semi auto that had an almost identical problem; the front trunnion was out of spec and the bolt was binding.

 

[DINK]

Since it just got rebarreled and you describe it as being very hard to open when a round fails to go off, my guess would be insufficient headspace. The bolt is barely closing in most cases and an occasional round is long enough to really lock things up.

 

[bruh44]

Either way, I’m sure I’m going to have to bring it back to the guy that did the barrel. I was just hoping it was something simple I could do. But it didn’t have this problem before the barrel got replaced. It does run 100% in auto.

 

[OldBesty]

Since you're an MG guy, there's good chance you haven't cleaned it yet...it could simply be due to the accumulation of fouling over the tighter tolerances of new barrel chamber from running FA, enough so that is preventing it running SA correctly.

Did you check appearance of the casings, particularly the stuck ones (they cay indicate if surface irregularities inside the chamber)? Have you gotten good look at the chamber? It could be needing light sanding and/or a good polishing job depending what you see.

Remember to inspect your spare barrels too. If headspace is excessively tight, might need to have them reamed.