after checking some of my mags and hearing some of the problems people are having with the 7.62x39 mags.
i thought i would double check mine and here is what i have found.
on the mags that wouldn't hold more than 27-28 rounds the mag spring was installed backwards-- this lets the nose of the follower dip /nose dive and causes a bind in the mag.
turning the spring around seems to pick up the nose of the follower and lets the mag hold a more proper 29-30 rounds.
i just opened two brand new in the wrap C-prod 7.62 30 round mags and the same thing--the springs were backward. they would not hold over 27 rounds and the follower would nose-dive.
these are the slick finish new i got from palmetto for $9 a pop back last year that don't have the no-tilt followers. so there the old style mag.
i turned the spring around to get the spring lifting the front of the follower and they now will hold 30 rounds. cycling the rounds through the mag by hand they seem much smoother.
on the mags i have been shooting all but one had the springs in correctly. -- i don't know if i took them apart and put them in correctly or if they came correctly assembled.
anyway try this and see if it helps if your having problems with a 7.62x39 mag.