Get Polish polymer 30 round mags. The lightest of all readily availavle 7.62x39mm mags. They fit well, and they are LIGHT.
For instance (don't laugh), TWO 30 round Polish mags (empty) taped together weigh less than ONE 30 round steel Polish mag (empty).
Polymer mags don't rust and weigh less... I reccomend them. DO NOT GET PROMAG MAGS THOUGH!