If you have another rifle chambered in 7.62x39 try the new crate in that rifle.It sounds like you have some crap ammo if everything else you've tried so far has worked.Your chamber could still be a bit on the tight side and the ammo your using now may be somewhat out of spec,and the combination of the two is what is causing the sticking.Check this thread out,I think you can use a 308 flexhone for your application
https://xcrforum.com/index.php?topic=9141.0