The only "bad" 7.62x39 ammo I have ever experienced, was the Iraqi 7.62x39 that was for sale several years ago.
Round on the left. I bought several hundred rounds of the stuff at a gunshow. I found out why it was
cheap. These rounds were so bad, they made Wolf ammo look like hand loaded Match ammo. Not only was it dirty, and had a very strange smell, and the powder charge from round to round was different. One round would fire like a +P+, the next would barely get the round out of the barrel. I pulled several bullets from cases that had hard primers, and discovered there was
no powder!!
:surfing