i have built a few 7.62x39s. they run pretty good. the big problem is mags the c-product mag is about all you can get these days. they generally don't run with a full load of rounds.
a few other little nit-picky deals like the firing pin can't strike deep enough to set off some of the russian ammo with deep set primers. you can tirim the back side of the bolt and get a slightly deeper strike.
the 7.62x39 is a little more involved that a 5.56 is --though it ain't all that hard either--LOL